A physical and corresponding mathematical model is developed to describe the kinetics of electrochemically or chemically (classically) promoted catalytic reactions. The model, which can be viewed as an extension of Langmuir-Hinshelwood-Hougen-Watson kinetics is based on electrochemical Langmuir-type adsorption isotherms which can account explicitly for the attractive or repulsive electrostatic interactions between the adsorbates and the "effective double layer" present at the catalyst-gas interface. The model, which contains only measurable parameters and leads to mathematically tractable rate expressions, predicts directly the recently established promotional rules of catalysis and is in good semiquantitative agreement with experiment regarding the dependence of catalytic rates on the partial pressures of the reactants and on catalyst work function.