The antioxidant properties of extracts of the red grape pomace of the Isabella variety (Vitis labrusca × Vitis vinifera) as the source of polyphenolic compounds and the product with potential economic return were studied. Criteria are formulated. The potentiometric method for analysis, incl. express control and extracts of grape pomace, is justified. Representatives of polyphenolic compounds, contained in the grape pomace, and their mixtures have been studied. The effect of some extraction parameters on the value of the antioxidant capacity (AOC) and the total content of polyphenolic compounds in extracts, namely temperature, extraction time, the presence of a grinding stage, and the extractant composition, was studied. The patterns and trends, established by the potentiometric method, are consistent with the results of the Folin assay. The AOC of the extract was (8.5 ± 0.1) mmol-eq/100 g in the selected conditions of the extraction, namely, the temperature of 80 °C and 5 h of extraction into the water solution without preliminary grinding. This indicates that the secondary winemaking raw materials are quite a valuable source of natural antioxidants. © 2023, The Author(s), under exclusive licence to the Institute of Chemistry, Slovak Academy of Sciences.