Extant research underlines the critical challenge for firms to rigorously and consistently evaluate their growing number of cross-sector partnerships for sustainability and suggests formalizing evaluation processes by introducing formal practices. However, empirical research is scant and inconclusive. This study aims to develop an empirically grounded understanding of how firms formalize the evaluation processes of such partnerships and of what drives this formalization, to complement the so far mostly conceptual literature. We inductively analyzed 31 semi-structured interviews with 33 experts from firms (...) and their partnering nonprofits and further secondary data on organizations’ partnerships. We contribute to the cross-sector partnership research by analyzing firms’ practices related to the formalization of the internal as well as the joint partnership evaluation process with nonprofits. We propose a conceptual framework to explain why firms implement different formal practices to reduce the dependency on individuals’ skills, experiences, or judgments in evaluating partnerships. (shrink)