Aim of the research Luxury itself is an ambivalent concept. While the sustainable activities of luxury brands may be welcomed by some consumers (Park et al., 2019), others see luxury and sustainability as inherently opposed. Furthermore, the literature suggests that the type of sustainable cause, whether social or environmental, may elicit different consumer responses to brand engagement (Janssen, et al., 2017; Muniz & Guzmán, 2021). Collaboration with a celebrity may further reinforce these negative perceptions by highlighting the strategic aspect of the approach (Chang, 2011; O'Donohoe, 2001). Indeed, we do not know whether these conflicting reactions are more likely to arise from brands' sustainable activities or rather from the mobilization of endorsement strategies. This leads to the following research questions: What are the antecedents of ambivalence in the context of sustainable luxury endorsement? Is the emergence of this phenomenon due to the commitment of the luxury brand itself or to the presence of the celebrity in promoting the brand's activities? Theoretical background Sustainable luxury and ambivalence As an ambivalent concept, luxury can satisfy both the conformist desires of some consumers and the snobbish desires of others (Kastanakis & Balabanis, 2014). In the case of sustainable luxury, the existence of conflicting attitudes among consumers is a frequently observed situation (Kapferer & Michaut-Denizeau, 2020). Paradoxically, luxury brands can be perceived as sustainable not only in terms of the dimensions of product quality, durability and rarity (Kapferer & Michaut, 2015), but also through the ethical values of celebrities who support the sustainable practices of brands in the sector (Park et al., 2019). Commercial motives as a source of ambivalence Beyond the ambivalence generated by luxury itself (Dubois, et al., 2001), the endorsement of sustainable luxury can also facilitate the emergence of negative reactions. Celebrity endorsement can counterbalance the perception of incompatibility between luxury and sustainability, thus reinforcing positive consumer reactions (Muniz & Guzmán, 2021; Park et al., 2019). Conversely, the endorsement strategy can generate negative perceptions, as it resembles a consistent effort on the part of the brand to present itself in a sustainable light, thus making consumers skeptical (Chang, 2011). Methodology and key findings Exploratory qualitative phase In order to identify the existence of contradictory reactions at the origin of the ambivalence phenomenon, the AOL technique was first used. The AOL is a projective technique already used in luxury research (Hemonnet & Valette-Florence, 2020), which allows to uncover hidden motives or thoughts related to a given consumption context. In this study, participants were asked to imagine that an influential figure had endorsed the sustainable commitment of a luxury brand of their choice. In addition to examining the effect of the endorser on participants' perceptions, we also distinguished between two types of cause: a social cause, illustrated by a donation to a non-governmental organization (NGO) (the "Donate" group, 6 respondents), and an environmental cause, illustrated by the integration of recycled materials into the brand's supply chain (the "Recycling" group, 6 respondents). The survey followed the three usual steps mentioned in the literature (Kessous & Valette Florence, 2019), relying on an INDSCAL approach to derive the AOL mappings. Those mappings revealed two notable findings. Firstly, the coexistence of positive and negative responses suggests a phenomenon of ambivalence in support for sustainable luxury. The antecedents are, on the one hand, a positive perception of the brand because of its commitment and, on the other hand, the perception of commercial motives. The study also suggests that consumers may react differently depending on the type of brand commitment (i.e. social vs. environmental). Empirically, in a second quantitative step, our research model manipulates the effect of celebrity endorsement and the type of cause on the emergence of positive and negative reactions in the endorsement of sustainable luxury (2 by 2 experiment). The research design was tested using a PLS SEM approach, given the small sample size (N=192) and the possibility of overcoming a multinormal distribution. Our research model proposes to measure the effect of celebrity endorsement and the type of cause supported on participants' positive reactions (i.e., perceptions of luxury associated with the brand [H1+] and intention to recommend the brand [H2+]) and negative reactions (i.e., commercial motives [H3a- and H3b-]). As suggested by the AOL study, commercial motives are perceived as moderating the effects. The coexistence of these conflicting reactions may thus explain the impact of a celebrity on the emergence of the ambivalence phenomenon resulting from the endorsement of sustainable luxury. Overall, the PLS model was validated according to the usual tests of convergent and discriminant validity and reliability, along with good fit indices (GoF = 0.366 and SRMR = 0.069). When the celebrity supports a luxury brand's advocacy for a cause, the positive effects on luxury perception (β = 0.451) and intention to recommend the brand (β = 0.520) show that hypotheses H1 and H2 are supported. Regarding negative effects, celebrity endorsement induces a negative moderation of commercial motives on the relationship between perceived compatibility and luxury perception (β = - 0.169), but this effect is not significant on the relationship between perceived compatibility and intention to recommend. Hypothesis H3a is therefore supported for this type of cause, but H3b is not. More specifically, the experiments show that celebrity plays a role in the emergence of the ambivalence phenomenon, especially in the case of a social cause. For an environmental cause, celebrity endorsement tends to have a stronger impact on the intention to recommend than in a situation without endorsement. The effect on luxury perceptions is similar whether the celebrity endorses the brand or not. For a social cause, celebrity endorsement has a stronger effect on the perception of luxury than in the absence of endorsement. It also has a positive effect on the intention to recommend compared to a situation without celebrity endorsement. This suggests that although celebrity endorsement of social luxury generates greater ambivalence, this type of cause also increases consumers' positive reactions more strongly.