The way the syntax of our first language (L1) interacts with the syntax of a language we are trying to learn (L2) remains a much debated issue in the field of SLA. Some of the possible facilitating factors include the presence of similar structures in the L1 and the saliency of the morphosyntactic structure under scrutiny in the L2 (MacWhinney 2005). In this study, we focused on a structure that contrasts these two factors: ERP responses to morphosyntactic violations in the past tense in polar questions in French learners of English with the auxiliaries DID and HAD were obtained. The morphology of past tense in these questions works similarly in French and English with HAD but not DID; however, violations are phonologically more salient with DID. 26 intermediate French learners of English listened to 192 simple polar questions, half of them containing the auxiliary DID (DID Condition) and half HAD (HAD condition). Half of the sentences in each condition were made incorrect by varying the presence of the past morpheme. 120 sentences containing other agreement violations and 120 sentences containing a semantic violation were added as fillers. Participants were asked to focus on the meaning of the sentence and evaluate its semantic acceptability. They also completed a timed Grammaticality Judgment Task (GJT) with similar stimuli and additional fillers. Behavioural measures in the GJT showed that the participants’ d’ was marginally better in the Had condition but their response time was shorter with DID. Violations in the DID condition elicited a P600 as well as a positive peak in the 300-500ms window, resembling a P3 component. The DID violation being the most salient one, this is consistent with the hypothesis that the P600 reflects, as the P3 does, the subjective salience of the stimulus (Sassenhagen et al. 2014). Violations in the HAD condition elicited a negativity in the 300-500ms window that was not limited to anterior sites, thus more reminiscent of a N400 than a LAN. N400 effects have been found to be elicited by morphosyntactic violations even in native speakers (Tanner & Van Hell 2014). Polar questions with DID represent a complex L2-specific structure, since they involve the movement of the inflectional morpheme from the main verb, where it would be in a declarative sentence, to the auxiliary. This represents an additional processing cost; yet participants were faster to decide for these sentences. This apparent discrepancy, as well as the presence of the P3, suggests that the P600 effect observed here in the DID condition is not a reflection of a better perception of the morphosyntactic error at hand but of an explicit reaction to the superior saliency of this violation. In the HAD condition however, it seems that violations were not perceived as subjectively salient events but as lexical violations. These results suggest that in the absence of similarity with the L1, other cues such as the phonological salience of the violation are used to process morphosyntactic violations. These findings also have theoretical relevance since they strongly support the P600-as-P3 hypothesis.