ReGrow4C aims at substantially extending the life span of peat- and perlite-based growing media. The extraction of peat from pristine peatlands threatens these sensitive ecosystems and carbon sinks, meanwhile resulting in increased emissions of greenhouse gases. Each batch of peat that is reused as growing medium results in a clear reduction in CO2 emissions and a lower impact on the climate. After using growing media for one cultivation, we aim to reuse the spent material as substrate for another crop. Spent peat and perlite based growing media from strawberry and cucumber cultivation were tested for upcycling after steam treatment. The first focus was on assessing the stability of the spent growing media, in comparison with values for a range of composts. In this study C mineralization was assessed by CO2 flux measurements in the lab during a 30 day period. We compared the CO2 flux measurements with four other indicators for assessing stability of the materials: Oxygen uptake rate (OUR), biodegradation potential, mineral N content and risk for N immobilization. The spent growing medium had a low decomposition rate, and CO2 flux measurements were found to be applicable for stability assessment for spent growing media and composts.