Students' reflective essay writing can be stimulated by the formative assessments provided to them by their teachers. Such assessments contain information about the quality of students' reflective writings and offer suggestions for improvement. Despite the importance of formatively assessing students' reflective writings in teacher education programmes, it is difficult for teacher educators to provide high-quality formative assessments. In this article, a quantitative content analysis procedure (QCA) is used to assess the level of argument and content of 34 student teachers' reflective writings over the course of two semesters. The study showed that the mean argument levels of students' reflective essays differed between the two consecutive semesters. The modes of the reflective essays were primarily descriptive and evaluative. The results indicated that it is important to encourage students to focus on the content of the justification, dialogue and transformative learning in their reflective essays. It is concluded that QCA can be used as a procedure for teacher educators to formatively assess their students' reflective writings, reliably and validly.