This poster presents a study of the interaction between the word structure of words beginning with a preposition and the grammaticalization of these prepositions into a prefixes. This interaction will be applied to Dutch and French data. In line with the studies of Amiot (2004, 2005 among others) on the subject, I claim that the boundary between preposition and prefix is not strict, but that prepositions can undergo a grammaticalization process and end up as true prefixes. Since prepositions, on the one hand, typically act as the Head of a prepositional phrase (Melis 2003) and prefixes, on the other hand, act as Modifiers of the stem to which they are bound, the "prefixization" process has to turn a prepositional Head into a prefixal Modifier. This grammaticalization process is in direct conflict with the typical word structure of French compounds. Indeed, French compounds are structured by the Head-Modifier principle: in compounds as timbre-poste (litt. 'seal-post'  'stamp'), the Head timbre precedes the Modifier poste. We hypothesize that French compounds beginning with a preposition will often comply with the Head-Modifier principle and will generally give rise to "exocentric compounds", that is compounds in which the non-prepositional component does not act as the semantic or morphological Head (e.g. entrecolonne 'distance between two columns'). These exocentric compounds are still very close to prepositional phrases and show a "syntactical" word structure. Dutch compounds, by contrast, typically comply with the inverse word order Modifier-Head (e.g. postzegel (litt. 'post-seal' → 'stamp')). If we apply this word structure on words beginning with a preposition, it will rather stimulate than counteract the prefixization process and will lead to more morphologized construction types, such as "(semi-)endocentric compounds" in which the non-prepositional component is the Head of the word and the prepositional component acts as a Modifier (e.g. tussengeneratie 'intermediate generation') and "derivations", ...