The purpose of this study is to investigate an under-researched area of adult learning in informal and unstructured online spaces. The first phase of the project involved a systematic review of 31 studies on adult learners’ peer-to-peer (P2P) interactions in online learning environments. The aspects explored were: (1), the characteristics of adult P2P engagement in online collaborative learning environments; (2), the impacts of that engagement on the learning outcomes of adult learners; and (3), the factors that could facilitate or hinder adult engagement in such environments. The review revealed that most studies investigated the broad effects of P2P adult learning on learning outcomes. These effects suggest that: (1) the adult learning efficiency could be improved through the application of more specialized approaches; and (2), that various unexplored factors may be important in facilitating P2P adult learning. This research will allow for better consideration of adult learning processes and activities.