This study examines the impact of group composition (one-on-one vs. multiple-to-multiple) and task design (student-selected vs. teacher-assigned) on undergraduate foreign language learners' interactions in a mobile-based intercultural exchange. The participants, 27 Korean students learning English as a foreign language and 27 American students learning Korean as a foreign language, interacted in pairs and groups via mobile phones to complete weekly tasks for eight weeks. This study used mixed methods to analyze the data from mobile chat scripts, questionnaires, and interviews. The results indicated that the one-on-one and multiple-to-multiple groups did not differ significantly regarding contact frequency or number of written chats. However, one-on-one and multiple-to-multiple interactions did differ with regard to the quality of the interactions, reflecting the unique nature of each group composition. A one-on-one relationship promoted a higher level of intimacy and friendship, thus rendering it appropriate for providing linguistic and emotional support in learning foreign languages. In contrast, multiple-to-multiple communications were found to be more beneficial for learning about different perspectives on the target cultures. In terms of task design, teacher-assigned tasks guided students to engage in productive interactions effectively, whereas student-selected tasks elicited their personal investment in the tasks. Supporting social interdependence theory (Johnson & Johnson, 1989, 2009), we argue that the establishment of intimate relationships among group members may be the key to quality interactions in mobile-based intercultural exchanges.