Even though collaborative prewriting tasks are frequently used in second language (L2) writing classes (Fernández Dobao, 2012; Storch, 2005), they have not been as widely researched as other tasks, such as collaborative writing and peer review. This article examines the effectiveness of collaborative prewriting tasks at encouraging English for Academic Purposes (EAP) students to engage in critical reflection while brainstorming the content and organization of written texts. Drawing upon data from three experiments (Neumann & McDonough, 2014a, 2014b), the impact of task design and students' perceptions about collaboration on their prewriting discussions are explored. Suggestions for instructors with an interest in using collaborative prewriting tasks are provided.