A lumped pseudokinetic model that is able to predict the toxicity change during the treatment of industrial wastewater as a function of the initial toxicity and temperature of the process is proposed. The original pollutant (or pollutant mixture), P, reacts to form a lumped intermediate species, I, which, in turn, reacts to form a nontoxic lumped species D. Therefore, only P and I in this serial scheme, P -> I -> D, are considered to contribute to the toxicity of the aqueous sample. The median effective nominal concentration of species I, EC50(I), is a fitting parameter in this lumping approach and gives an idea of the toxicity of the first organic intermediates generated, lumped as I. The model was successfully validated by analyzing the toxicity data obtained during the catalytic wet oxidation of phenolic solutions (with phenol, o-cresol, and p-cresol employed as pollutants) in a three-phase fixed-bed reactor. Moreover, the lumped kinetic model developed in a previous work for the mineralization of phenol was also validated by analyzing the total organic carbon (TOC) evolution data obtained with o-cresol and p-cresol as pollutants.