This quasi- experimental study aims to investigate the effect of synchronous computer-mediated communication (SCMC) on the development of learners' lexical skills in a way that increases their vocabulary repertoire and enables them to express themselves more effectively. Two intact groups of students, studying in an advanced module in a private university in Cairo, are assigned synchronous online chats and face-to-face discussions to negotiate the meaning of 12 new lexical items in context.  A pre and posttest design is employed to investigate the effect of SCMC on the experimental group in comparison to the face-to-face group.  A delayed posttest is administered to test for vocabulary retention. Results show that SCMC is more successful in promoting vocabulary acquisition and boosting retention of the acquired vocabulary items.