Alumina displays low catalytic activity at the initial stage of the reaction of CCl2F2 with hydrogen, giving mainly halogen exchange products, and this activity quickly decays with time-on-stream. In the case of Pd/Al2O3 catalysts, the contribution of the support is negligible at 180 degrees C. Catalytic activity of Pd/gamma-Al2O3 in CCl2F2 hydrodechlorination strongly depends on metal dispersion : poorly dispersed Pd samples exhibit the highest turnover frequencies. The same samples also show the highest selectivities towards the formation of CH2F2. Time-on-stream behaviour and considerable amounts of carbon found in used catalysts suggest that the catalytic properties of Pd/Al2O3 are regulated by incorporation of carbon into Pd lattice, or the formation of Pd carbide. Poorly dispersed Pd catalysts contain a higher proportion of plane atoms and, therefore, are subjected to a more severe carbiding. Such a transformation generates surfaces which bind freon molecules less strongly, resulting in higher activity and selectivity to partial dehalogenation, i.e formation of CH2F2.High-temperature reduction at 600 degrees C does not much change the overall activity of Pd/Al2O3. However, the selectivity to CH2F2 is somewhat increased. It is believed that the Pd-Al2O3 interface changes upon high-temperature reduction, leading to a Pd-Al compound. At corrosive conditions of hydrodehalogenation of CCl2F2, the Pd-Al would be converted to AlFx species much more easily than the Al2O3 species at the Pd-Al2O3 interface of mildly reduced Pd/Al2O3 catalysts.