Inability to produce coherent and content-rich expository essays is a major setback for English department students' academic performance. Apart from poor accuracy, poor quality of content and organization of ideas were the major problems. The root of these problems was apparently the lack of feedback, particularly from peers with different levels of proficiency. To cope with the problem, peer review practice was integrated into expository essay writing course to allow transfer of knowledge and strategies among students. To further investigate how the practice was carried out by mixed-proficiency dyads, the current case study focuses on observing the improvements occur in four sophomore students' expository essays during the first half of the semester. The participants were selected as they were the only dyads with a significant difference in writing proficiency levels. In addition to describing the improvements, the study also look at the nature of feedback given by those students. It was eventually revealed that improvement occurs in organization and content aspects, is surprisingly less significant compared to the improvement in vocabulary and language use. Responding to the findings, further research on this subject are still required to throw lights onto the efficacy of peer review for students with different characteristics.