Selective oxidation of cyclohexane and adamantane was performed by simple V(5+) catalyst with 1 atm O-2 in acetic acid at 338-393 K. UV-vis and electron spin resonance studies suggested that peroxide species on V(5+) functioned during the oxidation. The oxidation activity was enhanced three times for cyclohexane and 6.5 times for adamantane by addition of a small amount of CF3SO3H.