Three unique physical characteristics of diesel exhaust and their ramifications with respect to the diesel oxidation catalyst were discussed. The characteristics investigated were, first, that diesel exhaust contains solid, carbonaceous particles that can be catalytically removed from the exhaust only to the extent that they are first deposited within the oxidation catalyst. Calculated deposition rates for a certain 200 cell/in(2) catalyst substrate indicated that particle deposition and, hence, catalytic particle removal were negligible on a mass basis. Second, diesel exhaust hydrocarbons are composed of molecules much larger than those found in gasoline engine exhaust, There is consequently a maximum removal efficiency imposed by diffusion mass transfer limitations, calculable using classical mass transfer models, which becomes important at higher temperatures. Finally, diesel hydrocarbons adsorb onto the solid particles as the exhaust is cooled and diluted. Of particular interest is the state of adsorption at catalyst operating temperatures. Use of the Ideal Adsorbed Solution Theory indicated that adsorbed hydrocarbons account for less than 2% of the total particulate mass in undiluted exhaust at temperatures above 225 degrees C. Discussions of all three aforementioned topics are accompanied by experimental results, which in each case are consistent with the theoretical findings presented.