In the recent literature a few articles have been dedicated to the study of comparative modal constructions (or CMCs) in a number of Germanic languages: notably English, Dutch and German. However, CMCs are not restricted to the Germanic area but are also attested in Romance languages, for example in French. The present paper presents original data from three CMCs from French: faire mieux de, valoir mieux and falloir mieux (literally "do better of", "be worth better" and "must better"). The aim is two-fold. The paper first intends to offer a linguistic description of these CMCs: on the basis of corpora analysis, we give a morphosyntactic characterisation of French CMCs in synchrony, investigate their development in diachrony and describe their semantics. Along with this synchronic and diachronic description, the aim of the paper is also to define the degree of grammaticalisation of French CMCs; this ultimately enables us to specify their status of modal auxiliary.