Mixed richness communications occur when a participant in a conversation receives a different media or combination of media than they transmit. Mixed richness communications occur in the workplace when technical, physiological or practical limitations prevent the use of the same media on both ends of a conversation. Prior research in CMC has focused on same-richness communications, and the design guidelines that are available for same-richness communications may not be applicable to mixed-richness communications. This study attempts to establish a basis for understanding mixed-richness communications by evaluating same-richness communications using concepts and measures previously applied to mixed-richness communications Media Richness Theory (Daft & Lengel, 1984, 1986) defines the richness of a communication medium in terms of its ability to reduce uncertainty and equivocality. According to Daft and Lengel's task-media fit hypothesis, communications are most effective and satisfying when the media richness matches the level of uncertainty and equivocality in a task. Social presence is the perceived ability of a medium to transmit the social cues that lead to a sense that the medium is "warm, personal, sensitive, and sociable" (Short, Williams, & Christie, 1976). Social presence has been suggested to be a predictor of user satisfaction for computer-mediated communications (CMC), and has been used as measure of media richness in previous studies (Rice, 1993; Yoo & Alavi, 2001). This study examined the effects of communication medium and task equivocality on task performance, communication effectiveness and sense of social presence. Pairs of participants were required to complete high and low equivocality collaborative tasks while communicating with each other using CMC. The communication media varied between participants. During some sessions, participants received and transmitted the same media (video-only or text-only). In other cases, participants transmitted text and received video or vice-versa. From the recorded transcripts of each user session was extracted task performance in terms of task time-to-complete and communication effectiveness in terms of the frequency of communication breakdowns. Based on the task-media fit hypothesis, it was expected that task performance and communication effectiveness would be affected by the interaction between communication medium and task equivocality. For the most part, task-media fitness was not confirmed. Only one of the four hypotheses supporting task-media fitness was confirmed for time-to-complete, and none of the four hypotheses supporting task-media-fitness was confirmed for communication breakdown frequency. In the overall analysis of time to complete, Medium was found to have had a significant effect. Sending and receiving text was significantly slower than all other tested media. Sending and receiving video was significantly faster than all other tested media combinations. After completing each task, participants completed a short questionnaire designed to measure the sense of social presence using the original scales developed by Short and Christie. The sense of social presence reported in video communications was significantly higher for all scales than the sense of social presence reported in mixed-richness environments. The sense of social presence reported in text communications was only significantly lower than mixed-richness environments for one scale, with no significant difference for all other scales. / Master of Science