The adjunct "en+Détquant.+Ndurée" ("en deux heures") is commonly used to bring out the perfective aspect of verbs. On that basis, the hypothesis is that the French preposition "en" conveys itself an aspectual perfective value. The observations on corpus bear out the hypothesis and point out a tendency to express short durations, and even zero duration -thus, the possibility for this construction to express the instantaneous aspect ("J'ai compris la situation en un éclair"). The aspectual value of "en" gives rise to a qualification of the syntactic subject's referent (according to Guillaume's hypothesis) throughout the short time which the agent takes in order to accomplish what the verb denotes ("Il a analysé la situation en moins d'une minute"). A further check on "spatial" ("Professeur en Sorbonne") and "notional" values adjuncts ("un élève en difficulté"), leading to consider that the aspect conveyed by "en" concerns the judgement passed on the subject.