In this paper we present the Meeting Mediator (MM), a real-time, personal, and portable system providing feedback to enhance group collaboration. Social interactions are captured using Sociometric badges [6] and are visualized on mobile phones to promote change in behavior. In a study on brainstorming and problemsolving meetings, MM had a significant effect on overlapping speaking time and interactivity level without distracting the subjects. Our system encourages effective group dynamics that may lead to higher performance and satisfaction. We envision MM to be deployed in real-world organizations to improve interactions across various group collaboration contexts. Keywords CSCW, Social visualization, Meeting support, Sociometric sensors ACM Classification Keywords H.5.3 Group and Organization Interfaces: Computersupported cooperative work