Based on their positive experiences with intra-organizational enterprise social software (ESS), the first organizations are currently deploying ESS in an inter-organizational context. Nevertheless, hardly any research has addressed aspects pertaining to the commonalities of and differences between ESS and existing forms of inter-organizational information systems (IOS). Following an information-processing view, and based on a systematic literature review, as well as on the results of an exploratory interview study, we propose a conceptual model of inter-organizational ESS usage and relevant usage determinants. Some of these are known from prior studies, but have not yet been applied to an inter-organizational context (e.g., trust, knowledge sharing, security), whereas others were newly identified in our interview study (e.g., confidentiality, productiveness, dynamics). The proposed model extends the current understanding of IOS and helps address the field of inter-organizational ESS usage more appropriately in theory and practice.