The oxidation of cyclohexane and adamantane catalyzed by Keggin-type transition-metal-containing heteropolycomplexes (TMC-HPCs) is reported. The reactions were carried out at 20 degrees C in CCl4 or in CH2Cl2 as reaction solvent in the presence of t-butylhydroperoxide (t-BuOOH) as oxidizing agent. A radical chain mechanism of oxidation by t-BuOOH in the presence of molecular oxygen is proposed for all complexes tested, involving a R-H hydrogen atom abstraction by an intermediate active form of the TMC-HPC. Under the examined conditions molecular oxygen was necessary to initiate the reaction, The preparation of Co(II), Fe(III) and Cr(III)-HPC samples and their composition under reaction conditions are also discussed, based on P-31-NMR and W-VIS absorption spectra.