To be effective, thinking skills interventions are likely to require specific fine-grain changes in the quantity and quality of teacher–pupil and pupil–pupil dialogue in the classroom, but these are difficult to establish. This study investigated the effects of collaborative philosophical enquiry over time on quantity and quality of interactive dialogue in 180 children aged 10 in 4 intervention and 2 comparison primary (elementary) mainstream classes in 6 schools in Scotland. Participating teachers received initial and follow-up professional development. Matched comparison classes received regular teaching. Video recordings of teacher-led and pupil–pupil classroom discussions before, and 7 months into, participation in the programme were analyzed. These were related to a theoretical model of the process. Changes in intervention classes included increased use of open-ended questions by the teacher, increased participation of pupils in classroom dialogue, and improved pupil reasoning in justification of opinions. There were no changes in comparison classes. Variation in degree of change between intervention schools was evident. Implications for future research, policy and practice are outlined, particularly for enhancing consistency and expanding the programme