The importance of group discussion in facilitating learning in science is widely acknowledged. At the same time, it is recognized that in the social context of small groups, peers' discussion processes and their subsequent learning are influenced by factors other than students' conceptual understanding. Focusing on the social processes of knowledge construction in group settings, this article investigates the ways Greek secondary school students interacted in pairs and fours while discussing and attempting to explain simple physical phenomena. The study showed that students progressed significantly more in their physics reasoning after participation in fours than pairs. Moreover, the analysis of discourse in the different groupings suggested that the differences in progress were related to the more constrained modes of interaction of pairs. © 1996 John Wiley & Sons, Inc.