The goal of this thesis was to ‘develop a tool that offers insights in message framing for behaviour change to improve communication towards customers.’ By using an Design Based Research and Double Diamond approach constant iterations were made between theory and practice to develop a support tool that helps sustainable start-ups to design message frames that stimulate pro-environmental consumer behaviour change. Furture recommendations are to further develop the tool, test the effects, and stimulate more integrated research on the psychology of behaviour change, sustainable behaviour barriers, and framing designs.