An essential characteristic of design communication is the need to talk about things (objects and concepts). This paper concerns the development of a practical method to assist the iterative development of computer-mediated communication (CMC) systems that support talk by designers working co-operatively over distance. Previous approaches to the study of CMC systems limit the extent to which the value of new media and mechanisms can be explored. We have developed an alternative method to exploring CMCs based on shifts between communication resources during design activity, based on Scrivener's (2000) postulate that users in a communication environment satisfy their communication purposes by selecting from the available resources those most appropriate for their communication purpose. Such shifts reveal insights into the relative strengths and weaknesses of the communication channels. The paper outlines the rationale behind shift analysis, provides examples of the analysis and concludes by reviewing the steps needed to use this method in iterative CMC development.