Partnerships between sectors can achieve better outcomes than can be achieved by individual partners working alone. Trust is necessary for partnerships to function effectively. Mistrust makes partnership working difficult, if not impossible. There has been little research into partnership functioning factors that influence trust and mistrust. This study aimed to identify these factors in health promotion partnerships. Data were collected from 337 partners in 40 health promotion partnerships using a postal survey. The questionnaire incorporated multi-dimensional scales designed to assess the contribution of factors that influence partnership trust and mistrust. Newly validated scales were developed for trust, mistrust and power. Multiple regression analysis was used to identify the significance of each factor to partnership trust and mistrust. Power was found to be the only predictor of partnership trust. Power, leadership, and efficiency were the most important factors influencing partnership mistrust. Power in partnerships must be shared or partners will not trust each other. Power-sharing and trust-building mechanisms need to be built into partnerships from the beginning and sustained throughout the collaborative process.