Additional observations of the physical characteristics have been introduced to current monitoring of aerobic digestion of sludges coming from two wastewater treatment plants. New parameters have included determination of floc size and sludge specific surface. The internal floc structure was also examined with the use of microtome technique for slicing flocs into very thin sections. During aeration, considerable changes in floc dispersion occurred. Analysis of floc size distribution enables observation of regroupment of different sludge fractions and successive increase in frequency occurrence of the finest fractions, which usually hinder dewatering process. The decrease of average floc size has been accompanied by the increase in sludge specific surface. The time needed for “biological stabilization” has not occurred simultaneously with the time when physical parameters have been shaped in the best possible manner for further sludge treatment. The joint analysis of biochemical and physical properties including above mentioned parameters has enabled the assignment of aerobic digestion time more precisely. It has special meaning in pre-designing research as well as during the start-up of the aerobic digestion systems.