Digital product passports have been proposed as a policy instrument to enable decision-making throughout product life cycles in favour of a circular economy. However, due to nascent conceptualisation and weak industrial embeddedness, the contents of such an instrument are a source of uncertainty. Situated in a mechatronics context, this multiple-case study explores the data needs for digital product passports. Extant research reveals seven data clusters: (1) usage and maintenance, (2) product identification, (3) products and materials, (4) guidelines and manuals, (5) supply chain and reverse logistics, (6) environmental data and (7) compliance. To contextualise these clusters, interviews with three original equipment manufacturers (OEMs) as well as their respective customers, service partners, suppliers and third-party recycling companies were conducted. Through a survey, each specific data point was assessed in terms of importance, availability and sensitivity. The findings show differentiating needs for data across these actors, yet the exchange of data and its supporting infrastructure for closing resource loops remain at low maturity. Consequently, policymakers are recommended to roll out digital product passports in gradual stages, while industrial managers should proactively reconfigure data flows to account for decision-making in a reverse supply chain. Future research is encouraged to explore the use of digital product passports for decision-making in other industries.