In order to meet the strict future emission standards monolithic catalytic converters play a very important role. Although they have been extensively studied, still there are some areas that need to be further explored like the warm up period, during which the hydrocarbon emissions are relatively high. A single channel one-dimensional model can help evaluate the physicochemical processes occurring in the converter. In this study a decrease in the level of hydrocarbon emissions for propylene and propane during cold start are analysed taking into consideration the effect of both catalytic and homogenous reactions. The model proposed accounts for the combined effect of heat transfer, mass transfer and exothermic chemical reactions which are all strongly coupled.