An extended model for the conversion of hydrocarbons on bifunctional catalysts is proposed. It incorporates hydrogen spillover into the concept of bifunctional catalysis (spillover assisted catalytic effect, SPACE) and postulates a coexistence of two types of spilt-over hydrogen species at the surface. According to the proposed mechanism hydroisomerization is accompanied by a recombination and, therefore, consumption of activated spilt-over hydrogen. The extended model satisfactorily explains experimental results which were apparently in contrast to the classical bifunctional mechanism. It is exemplary applied to n-pentane hydroconversion on erionite.