A special experimental setup was used to study the equilibrium gas composition after interaction of propene with pure thin polycrystalline nickel films and nickel films precovered with hydrogen under ultrahigh vacuum conditions. Isothermal reaction mass spectrometry allows the analysis of the gas phase composition under equilibrium conditions. The analyzed gas phase consists of desorbed reaction products formed in the adsorbate phase after adsorption of propene on nickel films at 273 K. This technique was combined with thermal desorption spectroscopy and measurements of adsorption-induced changes in the electrical resistance of the polycrystalline films. After interaction with pure nickel films, in addition to propene, hydrogen, methane, propane, and butane could be detected as reaction products. The interaction of propene with nickel films precovered with hydrogen leads to a higher yield of butane and additionally to the formation of pentane. Thermal desorption experiments carried out after adsorption of propene at 77 K show only desorption of hydrogen, when the propene coverage was small. At higher coverages also propene desorbed.