In this study, a direct assay, a modified CUPRAC (Cupric Ion Reducing Antioxidant Capacity) method, is developed to determine transition metal ion (Cu(II))-catalyzed pro-oxidant activity of polyphenolic compounds, vitamins C and E, and herbal samples in the presence of proteins containing thiol groups. Since transition metal ion-catalyzed pro-oxidant activity of phenolics is usually initiated with the reduction of the metal to lower oxidation states (as a prerequisite of Fenton-type reactions), this method involves the reduction of copper(II) ions to copper(I) by polyphenolic compounds (simultaneously giving rise to reactive species), binding of the formed Cu(I) to egg white protein –SH groups, and liberation of copper(I)-neocuproine (Cu(I)-Nc) chelate (showing maximum absorbance at 450nm) by treating the incubation product with a neocuproine–ammonium acetate mixture. The proposed method is validated against atomic absorption spectrometric (AAS) determination of protein-bound copper and protein carbonyl assay of oxidative stress. The proposed assay is faster and more specific than the carbonyl assay, and uses low-cost reagents and equipment. Pro-oxidant activity (i.e. proportional to absorbance) varies linearly over a relatively wide range with concentration, as opposed to the reciprocal correlations (i.e. linear regression of 1/(pro-oxidant activity) versus 1/concentration) of other similar assays. The pro-oxidant activity order of the tested antioxidant compounds in terms of ‘Quercetin Equivalent Pro-oxidant Activity' (QREPA) coefficients is: gallic acid>epicatechin>quercetin≈catechin>α-tocopherol>rosmarinic acid>trolox>caffeic acid>ascorbic acid. •A direct colorimetric pro-oxidant activity assay was developed for polyphenols.•Cu(II)-induced pro-oxidant action of polyphenols produces reactive species and Cu(I).•Cu(I) binds to egg white protein thiols, and is liberated by buffered neocuproine.•Pro-oxidant activity of phenolics is proportional to copper(I)-neocuproine absorbance.•Pro-oxidant activity varies linearly with antioxidant concentration, and is additive.