The pH and timing effect on the relative capacity of antioxidant compounds to scavenge the ABTS [2,2'-azinobis(3-ethylbenzothiazoline-6-sulphonic acid)] radical cation (ABTS(.+)) expressed as the trolox equivalent antioxidant capacity (TEAC) is assessed for a wide range of antioxidants using a modification of the original ABTS assay at different pHs (4.6, 5.4 and 7.4). To study both fast and slow reacting antioxidants, a stopped-flow method based on a low cost laboratory-made system was used. For most of the tested antioxidants, total antioxidant capacity values were found to be a function of pH and time allowed for the reaction. Structurally similar compounds have the same time and pH-dependent behaviour even if they have significant differences in total antioxidant capacity values. The ABTS(.+) scavenging reaction rate was found to depend strongly on pH for p-coumaric acid, glutathione, BHT and albumin. Quercetin, gallic acid, (+)-catechin and (-)-epicatechin showed the higher activities at all pHs. The stopped-flow method can be utilized for screening of antioxidant compounds of unknown kinetics towards ABTS(.+) at different pHs and the results can be used to predict the total antioxidant capacity of structurally related compounds. (C) 2004 Elsevier B.V. All rights reserved.