Four samples of municipal sewage from 2 treatment plants were fractionated by vacuum filtration (1.2 and 1.6 Am pores), centrifugation (3500 to 12500 mu m) and coagulation with Al sulfate to sep. the soluble traction of pollutants. The content of readily biodegradable fraction was studied by respirometry for 3 h in presence of activated sludge. The fraction contained volatile fatty acids (19 to 73 mg AcOH/L of sewage).