The activation of C-H bonds of alkanes remains a major challenge for chemistry. In a series of deuteration experiments with D2 in contact with bis-(diphenylphosphino)butane (dppb) stabilized ruthenium nanoparticles (liquid substrates, 60°C, 6 bar D2) we have observed a surprisingly large reactivity of cyclopentane as compared to cyclohexane and other alkanes. DFT calculations using a ligand-free Ru13H17 model cluster as catalyst indicate oxidative C-H cleavage of the bound substrates as rate limiting reaction step. They also indicate similar binding and activation enthalpies of reactions of cyclopentane and cyclohexane.