Experimentation is an important capability in the transition to a sustainable business. The aim of business experimentation is to learn and improve business model innovation activities with limited risks and resources through continuous and collective learning with stakeholders. Through action research we worked with eight case companies on a pathway to becoming a sustainable business. We focused on 'circular economy' as a driver for sustainability. The process and role of business model experimentation were analysed. A circular business experimentation framework was developed and applied. We found that 1) experimentation creates internal and external engagement to start business sustainability transitions 2) experiments can help test assumptions in every building block of the business model 3) collaboration with external partners can ease experimentation, and 4) experimentation processes are iterative and require regular learning and sustainability checks. Further research is necessary to analyse how sustainability targets can be integrated into the experimentation process.