Abstract: This research explores the potential determinants of trust in newly formed virtual teams to better inform the choice of communication media for virtual emergency response groups for which interpersonal trust is crucial. Results of a laboratory experiment indicate that the way that communicants view their teammates and themselves as part of that team is significantly correlated with the level of trust that they experience. Impression formation, public self-awareness, perceived social presence, and self-disclosure are all found to have significant correlations with trust among ad hoc teammates. It is therefore recommended that the design of emergency response communication systems include features to foster these feelings thereby promoting trust among teammates. Our results are applicable to ad hoc emergency teams who interact for a period of time to address relief and recovery efforts after a major disaster or emergency.