Based on the distinction between expressive and instrumental motives, six theoretical mechanisms for the formation of trust relationships are elaborated and empirically tested. When expressive motives drive tie formation, individuals primarily attach emotional value to social relationships. Three mechanisms have been tested: the homophily, the balancing, and the gossiping effect. When instrumental, control-related, motives drive tie formation, actors strategically establish relationships because of their potential use for the realization of material benefits or the avoidance of material losses. Again, three mechanisms have been tested: the signalling, the sharing group and the structural hole effect. Longitudinal data come from a sociometric panel study of 17 members of the management team of a German paper factory. Actor-oriented statistical modelling shows that all effects significantly affect trust formation separately. In a simultaneous test incorporating all six mechanisms, the pattern of structural holes turns out to be the major predictor of network evolution. The implications of structural hole theory for modelling the evolution of intraorganizational networks are discussed.