Appropriate partner selection is a crucial concern for businesses who want to address material sustainability issues through cross-sector partnerships. However, the current research on materiality analysis and partner selection is still in the nascent stage of development. This paper contributes to understanding partner selection by corporates through an in-depth exploration of the business's selection of nonprofit organisation (NPO) partners, based on case study research of Northumbrian Water Ltd. (NWL), a UK private sector organisation providing water supply and sewerage services. Using the evidence from this study, we propose a framework for the selection of partners based on the prospective impact of cross-sector partnerships on material sustainability issues. Our paper also provides a useful toolkit for businesses to evaluate these prospective impacts. The framework demonstrates how materiality with respect to sustainability issues can be assessed both reactively and proactively, and how this assessment can better inform the partner selection process by corporates. This understanding of how materiality is assessed and partners are selected in practice is supported by a context specific exemplar and contributes to knowledge and practice of materiality analysis as well as cross-sector partnerships.