Reactions on catalytic surfaces can be represented by Langmuir-Hinshelwood (LH) mechanisms and, depending on the mechanism, the reaction rate has some dependence on the total concentration of surface active sites. The concentration of these sites is proportional to the surface area of the catalyst. The irregular surface of a catalyst can be modeled as a fractal and, as such? the total available surface area depends on the size of the reactant. In this paper the concept of fractal geometry has been extended to help discriminate between mechanisms for reactions of a homologous series on fractal catalytic surfaces. In the past, differences in rates for homologous reactants have been attributed to factors such as electronuclear induction and steric effects. These factors may indeed be important; however, modeling the surface as a fractal would explain the variations in reaction rate that could arise from changes in the reactant-size alone. The use of fractal analysis in this context is examined with reference to data on catalytic hydrodealkylation of alkylaromatic compounds. Simple plots used in this analysis (log initial rate versus log reactant cross-sectional area) are useful in discriminating amongst possible catalytic mechanisms. To verify the effect of reactant size on the reaction rate, a two-dimensional computer simulation is performed for catalytic reactions on fractal surfaces using reactants of different sizes. A LH mechanism is utilized where the surface reaction is rate controlling. The simulation is based on a Monte Carlo approach and considers the diffusion of reactants to a fractal catalyst surface, adsorption to and desorption from the surface, and surface reaction.