Investigations of processes occurring during wastewater treatment have progressed beyond the stage of technology. Currently, great numbers of representatives of diverse specialist research apply increasingly sophisticated measurement methods that have not been employed in this field of science. One of the methods is IRMS (Isotope Ratio Mass Spectrometry). Tracking changes in the ratios of biogenic element isotopes is useful in eg identification and monitoring of investigated processes. Since the IRMS technique has hardly been used for investigations of the wastewater treatment process, pilot research should be instigated to determine the isotope ratios occurring naturally in the process. The aim of the study was to determine changes in carbon and nitrogen isotope ratios at the successive stages of the technological line in wastewater treatment plants. The study material comprised: i) suspensions of raw sewage and mixtures of wastewater and activated sludge; ii) gases sampled from the volume of the suspensions; iii) gases sampled from the air above the suspension surface. The research material originated from the facilities of “Hajdow” municipal wastewater treatment plant in Lublin (SE Poland). The samples were analysed for the carbon and nitrogen isotope ratios, and the concentrations of the gases as well as total organic carbon (TOC), inorganic carbon (IC), Kjeldahl nitrogen (KN), dry weight, pH, and Eh were determined. The results obtained suggest that: i) the IRMS technique can be successfully applied in investigations of processes occurring during wastewater treatment; ii) isotope ratios in the carbon and nitrogen compounds (CO2 and N2) both in the suspensions and gases contained therein and in the air above them differ from each other and change at the different stages of the treatment process; iii) further research is indispensable in order to identify processes responsible for fractionation of carbon and nitrogen isotopes.