This paper investigates the power of collaborative learning in supporting students' task- processing in different learning contexts. By examining students' solo and dyadic activity in three learning environments, the study aims to unravel elements in the instructional settings and in the students' social and cognitive activity, which appear to support and, on the other hand, weaken successful task-processing and learning. Twenty 12-year old students from one Finnish elementary classroom participated in the study. Each student worked in solo and in dyadic conditions on three tasks located within the students' mathematics, language and science curriculum. The data were gathered by means of videotapes, direct observations, transcriptions, stimulated recall interviews, questionnaires and evaluations of the students' task productions. The social interaction emerging in peer groups was analysed at a micro-analytic level with the help of a three- dimensional analytic framework which focuses on the communicative function of peer interaction, and the nature of collaboration and of the cognitive processes taking place. The results of the study reveal situational mechanisms for meaning-making in collaborative learning. In addition, they highlight instructional conditions, which appear to give rise to useful and meaningful learning trajectories.