Our earlier experimental study has shown that exhaust unburnt hydrocarbon emissions from spark-ignition engines can be reduced effectively by using in-cylinder catalysts on the surface of the piston top-land crevice. In order to improve the understanding of the process and mechanism by means of which unburnt hydrocarbon emissions are reduced, a phenomenological mathematical model was developed for catalytic oxidation processes in the piston-ring-pack crevice.This paper describes in details the modelling of the processes of the gas flow, mass diffusion and reaction kinetics in the crevices. The flow in the crevices is assumed to be isothermal and at the temperature of the piston crown surface. The overall rate of reaction is calculated using expressions for mass diffusion for laminar flows in channels and a first-order Arrhenius-type expression for catalytic reaction kinetics of hydrocarbon oxidation over platinum.The model is capable of describing the time-dependent behaviour of the gas flow and catalytic reaction in the crevices, thus provides a useful and convenient means of examining the effects upon unburnt hydrocarbons originated from the crevices of various design and operating parameters, such as the catalyst surface temperature, crevice geometry and fuel composition. The numerical predictions show that the surface catalytic oxidation is governed primarily by the resident time and the reaction kinetics on the surface of the catalyst. The effect of mass diffusion upon the surface catalytic reaction is evident only at higher surface temperatures.