Addition of molybdenum to nickel catalysts has a favourable effect on their properties in the steam reforming of hydrocarbons. Some attempts were made to explain the mechanism of promoter action by determining the proper-ties Of such catalysts in hydrogenolysis and cracking reactions. With small amounts of promoter (less than or equal to0.5 wt.%) advantageneous changes in selectivity of steam reforming and disadvantageneous changes in n-butane hydrogenolysis were observed. The promoter does not affect practically the catalyst properties in n-butane cracking. The effect of molybdenum was compared with that of potassium promoter applied in the industry.