The aim of this study was to investigate aerobic biodegradability of phenol, resorcinol and 5-methylresorcinol and their different two-component mixtures using activated sludge sampled from the Kohtla-Jarve wastewater treatment plant. The degradation behaviour of phenolic compounds was investigated by respirometry. Non-linear regression analysis was used for determination of the kinetic parameters such as the maximum rate of oxygen uptake (V(O2, max)), the maximum rate of substrate bio-oxidation (V(max)) and the half-saturation coefficient (K(S)). Various kinetic models were tested to obtain the best curve fit. It was shown that the activated sludge degraded resorcinol and 5-methylresorcinol more slowly than phenol. Among the studied substrates phenol had the highest values of V(O2,max), V(max) as well as the ratio of V(O2,max) /K(S). Activated sludge had the highest affinity to phenol with the lowest K(S) value. Among all studied bi-substrate systems the highest V(O2,max) values were found for phenol (0.1 mM) - 5-methylresorcinol. As the kinetic parameters and short-term oxygen demands are functions of the compound undergoing biodegradation and the composition of the microbial community performing the degradation, therefore the results of this study have importance in explanation of effectiveness of wastewater treatment process and of the influence of polluting compounds on it.