A comparative study on the performance and on the perception of two groups of students involved in a peer review activity on the writing of chemical laboratory reports is reported. The first group participated in a compulsory activity, while the second one participated on a voluntary basis. This study was aimed at determining whether the overall activity was more effective when the students voluntarily decided to participate. The results of this study seem to suggest that the use of the peer evaluation method for students in chemistry is partially affected by how the activity is proposed, whether voluntary or mandatory. The student perception, on the other hand, seems to be strongly influenced by whether the activity is offered as an obligation or as a voluntary option. This aspect could therefore foster a deeper and longer beneficial effect on learning for those students participating voluntarily.