Teaching chemistry as a practice rather than as a mere collection of facts demands that teachers modify their practices, particularly their approach to formative assessment (FA). In this study, we investigated how teachers' FA practices changed as a result of their participation in a professional development program designed with a Chemical Thinking perspective. Four FA portfolio chapters were collected from 19 secondary school teachers over the course of a year. The analysis of the FA portfolio chapters gave insight into changes in teachers' FA practice in the areas of task design, purpose, and focus when evaluating student work. All teachers implemented changes in at least one of these areas of analysis, with about half of them creating FA tasks that demonstrated changes in all three dimensions. Changes in FA design were the most prevalent among participating teachers, shifting from tasks designed to explore the acquisition of knowledge to tasks that explored student reasoning with chemistry. On the other hand, changes in evaluation focus were the least common as most teachers centered their attention on the correctness of students' answers rather than on the nature of students' chemistry ideas. The results of our investigation point to areas in which chemistry teachers require substantial support to effectively use FA in their classrooms.