Volatile fatty acid-contg. liq. phases from fermented activated sludge and a MeOH soln. of PhH, PhMe and xylenes (BTX) (250 to 750 mu g/L) were added to a municipal sewage and to an artificial one to study the course of the 2-step, (anaerobia and aerobic) biolog. treatment of the sewage under lab. conditions by detg. COD, pH, dissolved O concn., Kjeldahl, ammonia and nitrate N contents as well as BTX concn. The addn. of BTX resulted in a decrease both in the efficiency of org. matter removal from the sewage (high COD) from 70 to 90% down to 35 to 70% and in efficiency of the nitrification of the sewage. In the BTX-contg. sewage, high ammonia N and low nitrates contents were obsd. after the treatment.