A method of and reagent for determining hydrogen peroxide, in which the hydrogen peroxide containing sample is treated with a colouring reagent comprising a peroxidase, a hydrogen donor and an electron or radical acceptor which react stoichiometrically with the hydrogen peroxide to produce a pigment which is then measured colorimetrically. In accordance with the invention the hydrogen donor is specifically a compound of the formula ##STR1## where R.sub.1 is CH.sub.3, C.sub.2 H.sub.5, CH.sub.2 OH, CH.sub.2 CH.sub.2 OH, CH.sub.2 CH.sub.2 NH.sub.2 or COCH.sub.3 ;R.sub.2 is CH.sub.2 OH, CH.sub.2 CH.sub.2 OH, CH.sub.2 CH.sub.2 NH.sub.2, CH.sub.2 NHCOCH.sub.3, CH.sub.2 CH.sub.2 NHCOCH.sub.3 or COCH.sub.3 ;R.sub.3 is H, CH.sub.3, C.sub.2 H.sub.5 or OCH.sub.3 ; andR.sub.4 is H or CH.sub.3.The method and reagent may also be used in the determination of other materials such as cholesterol, phospholipid, glucose and uric acid, by initial treatment with an oxidase thereby quantitatively to produce hydrogen peroxide which is then determined colorimetrically by the method of the invention.