A study of factors affecting hydrocarbon oxidation in a diesel oxidation catalyst was undertaken. The objective was to determine whether interactions between particulate-adsorbed hydrocarbons and the catalyst significantly influenced hydrocarbon oxidation. Theoretical modeling supported by experimental data obtained at the U.S. Bureau of Mines' Diesel Emissions Research Laboratory indicated that the mass of particles interacting with the ceramic support was negligible. Additionally, a model of hydrocarbon adsorption onto diesel particulate predicted that over 98% by mass of exhaust hydrocarbons would be gas-phase, rather than particulate-adsorbed, at converter operating temperatures. A second physical process, the diffusion of gas phase hydrocarbons to the catalytic surface, was subsequently investigated. Theoretical and experimental results for the unburnt fuel hydrocarbons indicated that hydrocarbon oxidation was diffusion limited under high temperature operating conditions.