Previous research into enhancing children's educational dialogues during group work has recognised the role of teachers in modelling dialogue and guiding their students' engagement in reasoning. Whole-class plenaries also offer teachers such opportunities but the technological support of dialogue in plenaries remains relatively unexplored. The authors' comparative study evaluated teachers' use of Talk Factory--innovative software designed to model and represent students' engagement with the ground rules of exploratory talk in science plenaries. Their qualitative analysis compares the dialogue during lessons taught by two teachers where Talk Factory was used, with previous lessons by the same teachers where it was not used. They found that the nature of the dialogue in the intervention classes changed considerably: instead of plenaries characterised by unsubstantiated responses from students, the teachers used TF to mediate their students' exploration, and challenge, of each other's ideas. The authors explore the significance of these findings in respect of practice-based implications and future research. (Contains 3 figures and 1 note.)