Formulation of mandatory requirements in procurement projects has significant influence on opportunities for development and deployment of Open Source Software (OSS). The paper contributes insights on a widespread practice amongst public procurement projects which causes problematic lock-in effects and thereby inhibits opportunities for use of OSS solutions. Through a systematic investigation of 30 randomly selected procurement projects in the software domain the paper highlights illustrative examples of mandatory requirements which cause lock-in and presents five recommendations for how requirements instead should be formulated in order to avoid causing lock-in. Findings show significant lock-in caused by current procurement practices with a stark preference for proprietary software and SaaS solutions amongst procuring organisations.