This research identifies (using a systematic literature review), verifies (using in-depth interviews), and tests (using a large-scale survey) the relevant attributes of inter-organizational relationships. The research process was iterative and covered both desk and field research. Given the industry-dependent nature of relational attributes and the dynamics of inter-organizational relationships, the study was carried out in the fast-growing software industry in Poland, namely on expanding software developers (3 in-depth interviews & 786 sample surveys) and their long-term business partners (15 in-depth interviews).Interestingly, some of the relational attributes initially identified from the literature review (i.e., particularity, similarity, structure, and age of relationship) or verified as relevant (i.e., mutuality and power) appeared unreliable at the testing stage. Therefore, as our contribution, we offer empirically and positively tested multi-item measurements for fifteen attributes of inter-organizational relationships, namely: commitment, (lack of) conflict, communication, cooperation, formality, information exchange, investments, intensity, longevity, multidimensionality of bonds, quality, stability, transparency, trust and velocity.