Organizations from the not-for-profit (NFP) sector, such as social enterprises, increasingly engage in partnerships with other organizations – either within or across sectors – to increase the social value created (Montgomery et al., 2012). Today, there is a dominant, but not empirically tested understanding of cross-sector partnerships to be more effective in creating joint social value than within-sector partnerships as the former are argued to add more different perspectives and complementary resources to an alliance than the latter (Koschmann et al., 2012). However, cross-sector partnerships are often complicated due to different priorities, practices and values, whereas partners in within-sector relationships share similar thought patterns, thereby increasing effectiveness (Di Domenico et al., 2009). Overall, a precise and empirically proven understanding of the success factors enhancing joint value created is still due (Provan et al., 2007). Our study, hence, addresses the following research questions: (1) Does partners’ sector affiliation affect the partnership’s performance? (2) Which are relevant success factors for increasing joint social value created in NFP partnerships?