Constructivist learning theorists believe that it is important to encourage reflexivity, helping learners to think about how they approach problems, and how they look for and find solutions. A rationale is an explanation of the reasons underlying decisions, conclusions, and interpretations. In this study, requiring and supporting rationale sharing within the group is proposed as one way of promoting reflective thinking during collaborative learning activities. A collaborative learning activity was designed and implemented in an undergraduate project management course, requiring the documentation and sharing of decision rationales. A collaborative tool was developed to support the activity. The results of the study show that engaging in the process of documenting decision rationales potentially helped the participants to articulate their thoughts and ideas, effectively explore the course concepts, and engage in higher order thinking. Participants' feedback on the design of the collaborative space for rationale sharing was collected and design implications elicited.