International audience It is well known that Old French had developed a system of preverbs expressing aspectual distinctions. In our paper, we concentrate on two homonymic preverbs: (i) a preverb en-which originates from the Latin preposition in and (ii) a preverb en-originating from the anaphoric adverb inde. The analysis of a set of dictionaries and a diachronic corpus reveals that these two preverbs are on the decline in medieval French. This can be explained by the fact that the en-preverbs do not allow the construction of a transparent aspectual system, because the prefixed verbs and the verbal bases are polysemous and because this polysemy gives rise to variable aspectual interpretations. However, the decline of the aspectual in Middle French can also be seen in continuity with an evolution that already started in Late Latin (Haverling 2000, 2008, 2010), where the semantic changes affecting the imperfect and the perfect have neutralized the aspectual oppositions between certain bare verbs and the corresponding prefixed forms. In the last part of the paper, we look into the complex interaction between verbal and grammatical aspect and its influence on tense use.