The growing use of the Internet in learning environments has led to new models being created addressing specific learning domains, as well as more general educational goals. In particular, in recent years considerable attention has been paid to collaborative learning supported by technology, because this mode can enhance peer interaction and group work. Among the different active learning strategies, Cooperative Learning has found in the Internet and the World Wide Web the ideal technological support. In this scenario, we have implemented a web-based environment, endeavouring to reproduce within it the traditional teaching methodologies typical of cooperative learning. Herein we describe two experiments aiming to assess the quantity and quality of the interaction promoted by the system and how such factors as gender, background knowledge and role affect communication. We compared the communication and the learning gain achieved in the experimental group that worked with the system, named Geometriamo, with those achieved in the control group working in class with the teacher. The results demonstrate that the use of an environment mediating peer-to-peer collaboration can be highly beneficial, even in primary school age. A high number of messages were exchanged among all pupils and, notably, the highest learning gain was recorded in the less able students in the experimental group. (Contains 12 figures and 1 table.)