This study presents a modeling approach to the formation and growth of nucleation mode particles when sampling aerosol from the exhaust of a diesel engine. The simulation assumes primary particle formation due to sulphuric acid nucleation and subsequent particle growth as hydrocarbons condense on the primary nuclei. The modeling results are validated against experimental data of particle number concentrations at various dilution ratios and aging times after raw exhaust sampling. In addition, exhaust aerosol of different characteristics is produced by fitting in the engine exhaust line a diesel oxidation catalyst, a diesel particle filter, and a combination of the two. The model is able to satisfactorily reproduce the particle number and mass concentrations of exhaust particles in the nucleation mode for a variety of sampling and after-treatment conditions, with a small delay (50-200 ms) in the initiation of nucleation compared to the experiments. The measured and simulated particle concentrations are in the same order of magnitude with the exact simulated values, depending on the fuel-sulphur conversion rate and the profile of organics considered in the exhaust. This study indicates that a detailed chemical analysis of the exhaust gas, including organic speciation and sulphuric acid concentration, combined with an aerosol dynamics model, would result in a satisfactory prediction of the effect of after-treatment devices and sampling conditions on the exhaust aerosol characteristics. However, it should be stressed that modeling of diesel-exhaust processes is still in an early phase and further work is merited to better understand the processes and mechanisms involved.