Аннотация:Humic substances (HS) play protective functions by interrupting radical reactions and preventing damage to cell membranes due to their antioxidant activity. With HS, antioxidant capacity (AOC), rather than antioxidant activity, is usually used to describe antioxidant properties. Our work aimed at quantifying the main kinetic parameters of antioxidant activity for HS and humic-like substances (HLS). The ten-hour kinetic profiles of four standard HS and two fungi-produced HLS were established using the ABTS decolorization assay. Three pH levels (3.75, 4.25 and 6.80) and a broad range of humic material concentrations (0.5–10 mg L–1) were examined. Our results demonstrated that during the first 40 min, the determined AOCs did not exceed 50% of the endpoint AOCs for studies of humic materials, indicating that short-term measures of the AOCs of humic materials provide artificially low values due to the presence of slow-acting antioxidant compounds. The results showed also a clear-cut distinction in reaction rate constants and AOCs between HS and HLS. The difference thus discovered was visualized by cluster analyses, in which HS and HLS formed two groups.