Communication is frequently identified in the literature as a major factor impacting Information Technology (IT) project failure. The importance of communication is amplified in buyer - seller relationships through the long-term impact of project failures on the future business of IT vendors with their customers. The formal communication between IT project sponsors from buyer firms and project managers from IT vendor firms within business to business markets is investigated through this study. Typical communication patterns between project sponsor and manager in high and low performing projects are identified. The antecedents of these patterns are assessed and the effectiveness of project sponsor - manager communication investigated. A multi-method approach is used with a quantitative analysis of a worldwide survey with 200 responses, followed by a qualitative analysis of three interviews with pairs of project sponsor and manager, each pair from the same project. Results show that project sponsors expect more analytic and verbal communication from project managers. A model shows the development from frequent informal communication to formal communication between project managers and sponsors. A second model shows how communication in high performing projects is determined by the level of collaboration between project managers and sponsors, as well as the degree of structure in project execution. Effectiveness of project sponsor and manager communication is found to be decreased through written statements about recent achievements, and increased through face-to-face meetings of the parties. A series of recommendations is provided to improve project sponsor - manager communication.