The study used the method of principal component analysis PCA in order to achieve significantly correlated measurements online, showing the work of two nitrification reactors at the wastewater treatment plant in Trepcza in the period from 04.2014–04.2016. Separated three main components that range 74.3 % – 82.6% explained variability in the original data. It was found that the first main component, which to the greatest extent was associated with fluctuations of the oxygen concentration and ammonia nitrogen in the nitrification reactors, explained from 38.8% to 45.0% of the variability contained in the original variables. During the entire study period noted for nitrification reactor I independent of the half year (winter/summer) weak influence of temperature on the concentration of ammonia nitrogen. From April 2014 to April 2016 in the reactor and observed no correlation between the oxygen concentration and the concentration of ammonia nitrogen. In most of the test period – except in winter 2014/2015 year. In the reactor nitrification II was seen very strong correlation between oxygen and ammonia nitrogen. There has also been the effect of temperature on the reduction of N-NH4+ by their strong negative correlation.