Reactions of size-selected platinum cluster ions, Pt-n(+) (n = 1-5), with small hydrocarbon molecules (CH4, C2H6, C2H4, and C2H2) were studied in a beam-gas cell geometry at collision energies less than 1 eV employing a tandem mass spectrometer. It was found that dehydrogenation of the hydrocarbon molecules proceeds exclusively on Pt-n(+) and the absolute cross sections of the reactions depend critically on the cluster size, n, and the collision energy. The size- and the collision energy-dependent cross sections are successfully explained in an RRK framework.