Vygotsky's writings on Sociocultural Theory (SCT) of mind, his concept of Zone of Proximal Development (ZPD) and its related metaphor, scaffolding, serve as the theoretical basis for the study of peer collaboration. This paper aimed at examining the effects of peer-scaffolding on EFL writing ability and finding out how revising techniques are constructed and expanded when two learners are working in their ZPDs. Two intermediate EFL university students (a "reader" and a "writer") collaboratively revised a cause and effect composition written by one of them. A microgenetic approach was applied for analyzing the interactions. Results showed that even though the role of the reader was more significant as mediator at first, both reader and writer actively took part in revising the text with assistance transferring mutually between them at the end of the session. Results also indicated that peer scaffolding could be reciprocal rather than unidirectional. As a concluding remark, it was noted that teachers can enhance the quality of collaborative writing processes by helping students to work together and take into account joint efforts in the writing tasks. Learners can also take more participation, learn evaluative feedback strategies, and benefit from their co-mediators.