The issue addressed in this paper is the semantics of simple vs complex (or synthetic vs analytic) expressions, in particular that of complex prepositions in Medieval French. We present the categorial structure of prepositions in Old and Middle French, and go on to show, on the basis of a corpus study of a few simple and complex prepositions, that the latter are indeed less polysemous than simple prepositions, as one could await, but nevertheless demonstrate a certain degree of polysemy. Their semantic evolution includes, for some, leaps across conceptual domains – metaphorical transfers from space to time, for instance –, but it seems that it differs to a certain degree from that of simple prepositions, though this needs to be verified by further research.