The aim of this paper is to give an account of the emergence and evolution of the French « topicalization marker » à propos de (‘about’), as well as two other phrases : à propos and à ce propos. This study is corpus-based and deals with texts in Middle and Pre-Classical French (14th to 16th centuries). As we try to refine the chronology of the evolution, we show that all three expressions have always displayed both postverbal and preverbal (even initial) positions. While the expressions in postverbal position have always functioned as complements, strongly linked to the verb, those in initial position go through an important change : from verbal complement to topicalization or enunciative markers. As we explain in our paper, this evolution roughly corresponds to the three-staged movement described by Traugott in the global framework of grammaticalization, namely from a more objective to a more subjective value, or in other words, from a propositional to an expressive function, through a textual one.