Purpose The purpose of this paper is to identify the antecedent factors that foster and sustain the development of relational norms from a social exchange process perspective and articulates the mediating effect of relational norms on the relationship between inter-partner learning (IL) and IT co-sourcing performance. Design/methodology/approach A total of 197 usable questionnaires were returned from 1,000 Taiwan enterprises. Results from a partial least squares method supported the hypothesis that relational norms serve as dependent variable (to IL) and independent (mediating) variable (to IT co-sourcing performance). Findings The empirical results show that IL positively effects partnership identity and collaboration, while both significantly mediate the effects of IL on IT co-sourcing performance. Originality/value This study creates opportunities for further cross-disciplinary studies of inter-firm co-sourcing project especially with regard to relational norms and knowledge sharing.