Research Findings: This study examined the learning effects of collaborative group work under heterogeneous group composition among 5-year-old children, especially in terms of their social skills. To this end, the study utilized an experimental research design wherein 3 groups of differently composed dyads and a group of students who worked alone completed classification and perspective-taking tasks. The dyads consisted of a low-ability child and either a socially advanced, an intellectually advanced, or a (both) socially and intellectually advanced peer. First, we found that children with high ability and those with low ability in collaborative dyads had higher learning gains than singletons. This finding supports the benefits of collaborative learning for experts as well as novices and the effectiveness of mixed-ability groupings for young children's learning. Second, we observed that low-ability children working with socially advanced partners, even partners with low cognitive ability, showed significantly greater improvement in the perspective-taking task than those working with a cognitively advanced but socially less skilled partner. We discuss the findings in terms of the relation between executive function and social competence and emphasize the value of social skills in cognitive development. Practice or Policy: We propose suggestions for teaching practice and further research.