A simple vanadate anion VO3− does not catalyse various oxidations with hydrogen peroxide via the generation of hydroxyl radicals. The addition of pyrazinecarboxylic acid (PCA) dramatically accelerates the oxidation reactions (Shul'pin et al., 1993) via the H2O2 decomposition as confirmed by DFT calculations: the activation barrier of the HO˙ formation in the presence of PCA is by 9.6 kcal mol−1 lower than that in the absence of PCA. In contrast to the oxidation of methane in acetonitrile by the reagent hydrogen peroxide–vanadate anion–pyrazine-2-carboxylic acid (PCA), carrying out the process in an aqueous solution is accompanied by intensive parallel destruction of the co-catalyst (PCA). Therefore, relatively high yields of methane oxidation products (mainly formic acid) can only be achieved at a sufficiently high concentration of PCA. Additives of strong acids (sulphuric, trifluoroacetic and perchloric) increase the yield of products. It has been found that perchloric acid can also be used as a co-catalyst instead of PCA. Some strong inorganic and organic acids added to the aqueous solution noticeably increase the yield of formic acid as the main product of methane oxygenation.