Transposing ideas into correct, complex sentences is at the heart of writing competence in a school context (Saddler, 2019). However, the teaching of syntax and punctuation in French could be improved to facilitate the transfer of grammatical knowledge taught in class into free writing production (Nadeau & Fisher, 2011). In a quasi-experimental study carried out in an underprivileged environment with students aged 10 to 13, a teaching sequence comprising ten open sentence-combining activities (Saddler, 2019) was developed (Nadeau, Giguère et al., 2020b), then tested. To measure the effect of the sequence on the development of syntactic complexity, narrative and descriptive texts were collected before and after experimentation in 11 experimental and 6 control classes. The results show an increase in several indices of syntactic complexity for one of the grade levels. This article presents these results and the implications for teaching.