The goal of this contribution is to determine the phenomena involved in the formation of complex prepositions. We first discuss whether complex prepositions are to be considered a subclass of prepositions. A diachronic corpus study of a sample of such structures in Old and Middle French (a chief de lit. ‘at (the) head of’, a hore de ‘at the time of’, por l’amor de ‘for the love of’, au lieu de ‘instead of’, au-dessus de ‘above’) then enables us to show that lexicalization plays a major role in the first stages of their development. Grammaticalization appears only for a small number of already formed complex prepositions, which thereby acquire a more general meaning, with reduced phonological weight (e.g. du côté de > du côté > côté evolving from its original ‘lexical’ meaning ‘on/to the side of’ to its modern meaning ‘about’, ‘talking of’).