The challenge of providing authentic experiences and interactions for fostering oral proficiency and motivation in foreign languages is an opportunity for innovation in educational technology and instructional design. Although several recent innovations have received the attention of scholars, empirical investigation and validation is often lacking, as is the case with voice over instant messaging (VoIM). The goal of this study was to empirically test the effectiveness of VoIM for enhancing students' oral English proficiency and learning motivation. Different types of online discussions were examined--unstructured, structured without the facilitation of English teaching assistants (ETAs) and structured with the facilitation of ETAs. A pre-test and post-test quasi-experimental design was employed to achieve this goal. The participants, 90 students enrolled in three sections of a freshman English course at a large university in Asia, participated in eight VoIM discussions in addition to their regular coursework. The results provide empirical support for the importance of structured discussions and the facilitation of ETAs in improving both oral English proficiency and learning motivation. These findings highlight the role of facilitation in VoIM discussions, and recommend prompt feedback, modelling and encouragement for fostering oral proficiency and learning motivation in foreign language classes. Based on the results of this study, recommendations for future research are provided.