Triosmium dodecacarbonyl cluster Os3(CO)12 catalyzes oxidation of linear (n-heptane) and cyclic alkanes (cyclohexane, cyclooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexanes) to the corresponding cycloalkyl hydroperoxides by hydrogen peroxide in acetonitrile solution. Addition of pyridine leads to the acceleration of the process. Turnover numbers in the case of cyclooctane attain 60 000 and turnover frequencies are up to 24 000 h−1. The alkyl hydroperoxide partly decomposes in the course of the reaction to afford cyclooctanone and cyclooctanol. Selectivity parameters obtained in oxidations of various linear and branched alkanes as well as kinetic features of the reaction indicated that the alkane oxidation occurs with the participation of hydroxyl radicals. A similar mechanism operates in transformation of benzene into phenol and styrene into benzaldehyde. The system also oxidizes 1-phenylethanol to acetophenone. Glycerol is oxidized to produce dihydroxyacetone, glycolic acid and hydroxypyruvic acid.