This paper analyses the function of linguistic contexts in the grammaticalization process. Within the theoretical framework of the "evolution in three levels" (propositional level, textual level, pragmatic level), we underline the importance and the specificity of thematic constructions from a diachronic point of view. Certain expressions evolve first from linked adverbial to a thematic function, then to a topic function. This is exemplified by the study of two samples: the evolution of topic markers like quant à, en ce qui concerne, and the evolution of the subordinator dans la mesure où.