Endogenous processes such as biomass decay and intracellular polymers degradation of polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs) were investigated. Cultures enriched in Accumulibacter (a well known PAO) or Competibacter (a well known GAO) were subjected to 21 and 26 days of alternating anaerobic/aerobic conditions respectively. The main energy source for PAOs during starvation was their intracellular polyphosphate released into the medium during the first 14 days of starvation. In contrast, GAOs used their intracellular glycogen during the 26 days of starvation. Biomass decay rates were 0.029 d(-1) for PAOs and almost negligible for GAOs. The reduction in acetate uptake rate during the starvation period, referred to as activity decay, was 0.25 and 0.047 d(-1) for PAOs and GAOs, respectively. Once wastewater was reintroduced, both populations recovered their initial substrate uptake rate after 1 day. The results obtained show that PAOs are more affected than GAOs by starvation conditions. (C) 2012 Elsevier Ltd. All rights reserved.