Microbial activity measurements (oxygen transfer rate, OTR, carbon dioxide transfer rate CTR and respiratory quotient, RQ) are used to monitor and control biological processes. In this study, the digestion of activated sludge in microaerobic conditions was evaluated. The changes of biological activity during digestion were monitored by respiration activity. The sludge mixtures used were primary plus activated sludge, and baker's yeast plus activated sludge. The biological activity was triggered during the experiments by feeding sugar to the mixtures. The results show that increased activity by adding sugar to digested sludge can be monitored by OTR and CTR, whereas RQ can not be used as an indicator.