A special experimental setup and application of isothermic reaction mass spectrometry (IRMS) allow the analysis of the gas phase which consists of desorbed reaction products formed in the adsorbate phase after adsorption of acetylene on Fe, Ni, and Pd films at 273 K under ultrahigh vacuum conditions. This technique, combined with temperature-programmed thermal desorption spectroscopy (TDS) and adsorbate-induced changes in electrical resistance of the adsorbent film, was used to compare the activities of Fe, Ni, and Pd concerning C-C bond breaking, self-hydrogenation, dimerization, and trimerization of acetylene. As products methane, hydrogen, ethane, butane/butene, and benzene appear in the gas phase when the coverage with acetylene is increased. There is strong evidence that the products are formed already at low coverages. Additionally adsorbed deuterium leads to isotope exchange.