This paper examines a teacher's activity to reconstruct her choices and a part of her professional knowledge. Three dimensions of the teacher's activity are considered in this study: i) a cognitive dimension concerning the tasks given to the students and the chemistry content involved, ii) a mediative dimension regarding the unfolding teaching session, iii) a personal dimension including the teacher's knowledge and beliefs about chemistry and chemistry teaching. The teacher's activity in the classroom results from choices made during the design and the implementation of the lesson plan which depend among other on the teacher's knowledge and on the results of his/her actions. We assume that the comparison of two successive sessions of two groups of a same form provides indicators, modifications and unchanged features of the unfolding sessions, to infer pedagogical content knowledge (PCK) and pedagogical knowledge (PK). Our purpose is to determine them and to characterise the conditions and also PK that contribute to the development of PCK. The data are two video-recorded successive sessions of a 12 th grade form on the change criterion of chemical systems and of the teacher's interviews which took place just before and after the sessions. Several analyses follow: an analysis of the lesson plan to determine the tasks, of the unfolding teaching sessions to reveal the pedagogical strategies and the students' scaffolding, and of the teacher's comments to discover articulated knowledge. This first set of analyses provides tacit and expressed PCK and PK. The second analysis stage compares the two unfolding sessions and highlights the modifications that occurred, which are considered to be revealing the development of new PCK. A fine grain analysis of the chemistry content involved and of the classroom interactions allows to identify new PCK which also appear to be a consequence of PK involved.