During the period of April 3 through 8, 2011 Dr Daniela Šojic, asist. prof., Vesna Despotovic, associate researcher and Sanja Kler, junior researcher, stayed at the Institute of Material Sciences and Engineering, University of Szeged in order to continue the joint research in the frame of the IPA project. Bearing in mind that the degradation of metoprolol and thiamethoxam in the presence of O3 appeared to be faster in Danube and thermal waters than in the deionized water, the experiments were focused on the effect of different ions (chlorides, bromides and hydrocarbonate) present at different concentrations (100, 10, 0.1 and 0.01 mM) on the rate of degradation of the target compounds. The substrate degradation was followed by LC-DAD method.