The aim of our work was to determine the extent of inhibition of oxygen consumption by activated sludge for carbonaceous and ammonium oxidation (ISO 8192 2007) for various wastewaters. We have selected several types of wastewaters different in their origin and composition: pharmaceutical wastewater, tannery wastewater and municipal landfill leachate. To confirm results of toxicity testing, additional ready biodegradability assessment test with measurement of oxygen consumption was accomplished to indicate the impact of effluents to nitrification process. Pharmaceutical wastewater was toxic to activated sludge, but it inhibited heterotrophic microorganisms much more than nitrifying ones. Biodegradability testing confirmed low impact to nitrification by high, non-suppressed oxygen consumption for nitrification process. Tannery effluent inhibited nitrification significantly (180 min EC50 was 57 vol.%), but it did not affect heterotrophic microorganisms. Landfill leachate was very toxic to heterotrophic microorganisms (180 min EC50 was 3 vol.%), while it inhibited nitrification less (180 min EC50 was 24 vol.%). Presented research confirmed that the investigated experimental method is a reliable one for detection of occurrence of substances inhibiting nitrification in different industrial effluents. With regular monitoring of inhibitory impact biological treatment process upsets could be avoided and treatment optimised.