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Previously measured particulate mass concentrations from a single-cylinder indirect-injection diesel, obtained under conditions of both varying dilution ratio and varying filter temperature, are examined in detail. Considering the mechanisms of condensation, adsorption, and diffusion, the observed variations in total particulate mass are attributed primarily to the adsorption and desorption of exhaust hydrocarbons on the solid particulate matter. A simple Langmuir adsorption model is used to explain qualitatively the observed effects of dilution ratio and sample temperature. Only under conditions of relatively high hydrocarbon emission is the condensation mechanism also shown to be important. The simple adsorption analysis also predicts the trends observed in CVS (Constant Volume Sampling) dilution tunnels in which filter temperature and dilution ratio change simultaneously.