In the sharing economy market, B2B relationships between service providers and sharing economy platforms are largely built on the platforms' promise of ensuring a successful sharing experience, which is not always delivered. Moreover, platforms have the opportunity to take advantage of their dominant positions in the market. Despite the growing number of studies on the dark side of the sharing economy, little is known about psychological contract breach and opportunism in the context of B2B platforms. Building on these notions, this study set out to examine the determinants of individual service providers' relationship with a sharing economy platform using psychological contract theory. The study was conducted in the UK on a sample of 252 Airbnb hosts who were recruited through an online consumer panel. The results show that psychological contract breach is an important construct within platform B2B relationships which increases feelings of violation and reduces trust. Furthermore, perceived opportunism was found to be negatively related to feelings of violation. These feelings contribute positively to negative word-of-mouth, while trust leads to continuance intentions. In addition, the indirect effects of opportunism on negative word-of-mouth and of psychological contract breach on continuance intentions were confirmed.