This study analyzes the influence of avatars on social presence, interpersonal trust, perceived communication quality, nonverbal behavior, and visual attention in Net-based collaborations using a comparative approach. A real-time communication window including a special avatar interface was integrated into a shared collaborative workspace. Communication modes under investigation were text chat, audio, audio-video, and avatar. Significant differences were found between text chat and all other communication modalities in perceived intimateness, co-presence, and emotionally-based trust. Microanalyses of nonverbal activity and visual attention point to similarities between video and avatar modes, both showing higher levels of exposure to the virtual other and visual attention, in particular in the initial phase of interaction as compared to text and audio.