AhstractThis study investigated the significance of friendship for scientific reasoning. We had two main goals: First, assess whether collaborations between friends fostered greater development of scientific reasoning than collaborations between acquaintances, and second, identify which features of friends' collaborative dialogues mediated their greater problem solving accuracy. Fifth graders collaborated on isolation of variables problems of varying difficulty with either a friend or an acquaintance. The results showed that adolescents paired with friends obtained higher problem solving accuracy than adolescents paired with acquaintances during both the collaborative and the posttest sessions. However, the cognitive advantages of working with a friend were only evident in the most difficult isolation of variables problems. Analyses of friends' and acquaintances' dialogues showed that friends were more likely than acquaintances to evaluate their solutions, justify their proposals spontaneously, elaborate and critique each other's reasoning, and engage in transactive discussions. Fvaluating outcomes and participating in transactive conflicts were significantly associated with better problem solving, and consequently these may be the two key mechanisms through which friendship effects developmental changes in scientific reasoning. The results are discussed in terms of the need to understand developmental features of relationships and how these features mediate cognitive development and the need to take a longitudinal approach to the study of the relation between transactive dialogues and the development of"scientific reasoning.