Collaboration technologies offer the potential to enhance communication effectiveness, shared awareness, and decision quality in command and control (C2) application domains. The purpose of this experiment is to evaluate the impact of one of these technologies–Instant Messaging (Chat)–on team performance efficiency in a simulated human-in-the-loop air defence C2 scenario. An air battle management task required a team of two weapons directors (WDs) to communicate with each other, their fighter assets, and two refuelling tankers, to coordinate offensive counter-air, defensive counter-air, and air refuelling. The study compares the impact of three communications conditions (Voice-only, Chat-only, Voice and Chat) under varying levels of workload by manipulating the number of enemy targets (4 or 6) present in the battle space and the number of fighters (4 or 8) that the two WDs must manage. Team performance efficiency is assessed in terms of the proportion of friendly assets and enemy targets destroyed, the number of enemy target incursions into friendly airspace and the time required to eliminate the enemy targets. The results are discussed with respect to the efficacy of text–based collaboration technologies for supporting distributed team decision making in future C2 environments.