A first-principles-based kinetic Monte Carlo algorithm was used to simulate catalytic kinetics of ethylene hydrogenation on the well-defined Pd(100) surface. An intrinsic kinetic database was established from first-principles density functional quantum chemical calculations. This database was supplemented by an interaction model that was developed from 1750 extended Huckel calculations that explicitly examined adsorbate-adsorbate interactions between ethyl, ethylene, and hydrogen. Subsequently, this database was used to simulate ethylene hydrogenation kinetics via a multisite Monte Carlo algorithm that treats coverage-dependent activation energies through bond-order conservation methods. The simulated apparent activation energy for ethylene hydrogenation was found to be 9-12 kcal/mol, which compares well with experimental measurements of 6-11 kcal/mol. Kinetic orders vary with temperature and partial pressures of ethylene and hydrogen, but compare well with experiments under similar conditions. Simulated kinetic orders in hydrogen and ethylene are 0.65 to 0.85 and -0.4 to 0, respectively, under our conditions (298 K, P-H2 = 100 Torr, P-C2H4 = 25 Torr). The simulation results suggest that interactions in the adlayer, ensemble size effects, and adsorption site competition contribute to hydrogen kinetic orders that are less than one and negative orders in ethylene. (C) 2000 Academic Press.