The detection and determination of (a) hydrogen peroxide or of a substance which reacts with the formation of hydrogen peroxide or (b) of peroxidase or a peroxidatively active substance, is carried out by contacting an unknown sample with a chromogen and, in case (a), with a peroxidatively active substance or in case (b) with hydrogen peroxide or a substance which reacts with the formation of hydrogen peroxide, and evaluating the coloration produced by the reaction, wherein, as the chromogen, there is used at least one 3,3',5,5'-tetraalkylbenzidine compound of the formula: ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4, which can be the same or different, are straight-chained or branched alkyl radicals containing up to 6 and preferably up to 4 carbon atoms.