The study evaluated the role of endogenous decay and microbial activity in aerobic stabilization of biological sludge. Biomass acclimated to acetate in a fill and draw reactor operated at a sludge age of 8 d was subjected to aerobic stabilization with no external substrate for a period of 70 days. Achieved volatile suspended solids reduction remained limited to around 43% after 30 days. The magnitude of endogenous decay, presumably responsible for stabilization, was quite different with b(H) values of 0.21/d and 0.06/d depending on the implemented respirometric procedure. Model profiles simulated with these values could not predict the observed VSS profile, mainly due to the accumulation of particulate metabolic products and residual organic cellular debris of disintegrated biomass during the process providing conclusive evidence that the total organic content of the stabilized biomass cannot be reduced below a critical level by means of biological processes.