This article proposes the use of mathematical models obtained by the Pearson correlation between the concentration of various recalcitrant organic compounds (ROCs) measured by chromatographic analysis (ChrA) and experimental chemical oxygen demand (COD). The aim is to reduce the number of samples processed by the ChrA, diminishing the economic costs of analysis. Ten ROCs, including pesticides, colorants, aromatic hydrocarbons and pharmaceuticals compounds, were introduced into four advanced oxidation processes operated at different residence times. Every ROC was tested at each residence time by COD determination and by quantification of concentrations with ChrA. Furthermore, chemical equations for the COD reaction of every ROC were formulated. A linear model was obtained for all the ROCs, after corroborating that the correlation between theoretical and experimental COD was >0.99, which established the ROC concentration from the experimental COD, omitting the ChrA. Results indicated that it is possible to know concentrations in most of the ROCs by means of the experimental COD with a >99±0.01% of accuracy, which leads to a cost decrease and even to evaluate methods in developing countries, which often do not have chromatographs and where pollution issues are meaningful.