One of the most crucial problems in catalysis is the loss of catalytic activity. When analyzing or designing a reactive system involving a decaying catalyst, a rate law adjusted for the catalyst deactivation should be utilized. An example of deactivating catalysts is supported-iron catalysts used in the heterogeneous Fenton-like oxidations of organic compounds. In this study, the observed reaction rate of such reactive systems was modeled – through separable kinetics – with a general rate expression that was written in terms of a rate law, describing the reaction kinetics on the fresh catalyst, and an activity term, accounting for the catalyst deactivation. The model was developed and verified for the catalytic wet peroxide oxidation of selected dyes (Reactive Yellow and Red) over Fe-Y zeolite and drugs (Diclofenac and Naproxen) over magnetite/multi-walled carbon nanotubes, which experienced notable deactivation during the reaction. It was found that the reaction rate followed Langmuir-Hinshelwood kinetics while the catalyst decay rate was second order in the present activity and first order in the concentration of the fouling species, which presumably resulted from the degradation of the organic compounds. The effect of the temperature on the rate expression parameters was also studied. The model included parameters such as the reaction and decaying rate constants, reaction and decaying activation energies, adsorption constant, and enthalpy of adsorption. The rate expression proved to satisfactorily describe the concentration–time trajectories in all the studied reactions despite their differences in the utilized catalysts, reacting organic compounds, and/or conditions, which verified the robustness of the model