Establishing trust is a crucial yet significant challenge for peer-to-peer (P2P) carpooling platforms. This mixed-method study of trust-building mechanisms in carpooling platforms makes two contributions to this topic. First, a qualitative comparison of trust-building mechanisms in four platforms shows similarities regarding their reliance on review systems and contact opportunities, but differences regarding the use of GPS-tracking, the holding of offline events, and key partners, all of which represent potential trust-building measures. Second, a laboratory experiment (N = 163) suggests that these trust-building mechanisms have different consequences for car owners vs. non car owners and for more vs. less experienced customers. These findings allow us to discuss nuances regarding the development of trust in carpooling platforms, especially the difference between online trust, implicit trust, and reason-based trust. These insights indicate avenues by which sharing models, associated with the potential for more sustainable production, consumption and transportation, can be strengthened.