Constant magnetic field (CMF) was investigated for its potential to intensify chemical oxygen demand (COD) removal in wastewater when applied with a generalized Fenton reaction. Various types of industrial and domestic wastewater were tested in the experiment. A significant dependence was observed between the wastewater type, dose of reacting substances, time of exposure to CMF, effectiveness of COD removal, and the rate of the advanced oxidation process. The use of CMF resulted in the improvement of the treatment process; however, the efficiency depended to a great extent on the type of wastewater and characteristics of the contained organic matter. Domestic wastewater seemed to be most amenable to the applied treatment (82% COD reduction). The least reduction in COD occurred for wood industry wastewater. In the analyzed wastewater, intense oxidation was observed during the first 60 minutes of the process. Longer reaction time did not increase COD reduction, with the exception of wood industry wastewater. The experiment revealed the possibility of applying smaller doses of the Fenton's reagent, thereby keeping up unchanged treatment efficiency, provided that CMFs are applied.