This study investigated the effects of two types of audio channels upon the effectiveness of task-based interactions in a video-mediated context (VMC). Forty undergraduates completed a collaborative task (The Map Task) using either a full or half-duplex audio channel. Their performance was compared to face-to-face interactions, taken from the Human Communication Research Centre corpus of Map Task Dialogues. Effects of varying the audio channel were explored by comparing task performance, patterns of speech, and establishment of mutual understanding. Users of the full-duplex VMC made insufficient allowance for the VMC context; they completed the task less accurately than face-to-face participants, and interrupted each other more frequently than other participants. Participants in the half-duplex VMC however performed as well as face-to-face participants. They made sensible adaptations to the constraints imposed by the half-duplex VMC context, producing longer dialogues, with more explicit turn-taking management, and taking greater care in establishing mutual knowledge.