Company networks play an important role for SMEs. This is particularly the case for regional networks that are organized into specific clusters focusing on one particular industry. Within these clusters, companies join together to form co-operations that have varying intensities ranging from loose, sporadic co-operations to the complete integration of manufacturing and/or company processes. These co-operations are founded upon the willingness to trust and rely on one another, as well as refrain from short-term gain for the own company in order to achieve long-term, shared benefits for all partners. We argue that increasing intensity within these co-operations enhances the success of the participating cluster companies. Further, we argue that trust and written contracts mediate this relationship. The results of an empirical survey of 150 companies in the automotive industry that are part of four SME networks organized into clusters, indicate that the interpersonal trust perceived and experienced by the cooperating partners fully mediates the positive effect of the degree of integration and company success. In addition, greater interpersonal trust between cooperating partners is combined with less important written contracts, because trust evolves with the relationship.