Many tertiary-level courses assess students' participation in tutorial or online discussions. However, in educational and pedagogical research literature, criteria for assessing students' skills in engaging with peers remain unclear. This article describes an online assignment with a set of participation criteria and a method for assessing the quality of students' interactions with peers. The assignment focuses on students' ability to utilise their critical thinking skills while engaging with peers on a particular topic. This includes abilities such as responding to criticism, justifying one's view and contributing to discussion. While the assignment is designed for a critical thinking course in a philosophy curriculum, the method and participation criteria may be adapted for assessment in other discipline areas. (Contains 2 notes, 1 figure and 3 tables.)