A pure CO2 homogeneous condensation flow that has been experimentally studied by Ramos et al. is simulated with a new DSMC-based condensation model. Comparisons of the measured and simulated CO2 cluster size and number density distributions were found to agree well for low stagnation pressures. The same carbon dioxide homogeneous conden- sation model was then applied to the study of an expanding heterogeneous condensation flow of a 5% CO2 and 95% N2 mixture that has been studied at AEDC. A new kinetic-based model of N2 molecules condensing on CO 2 nuclei was developed using molecular dynam- ics techniques and the model was implemented in the DSMC simulation of the expanding mixture. It was found that incorporation of the heterogeneous condensation process of N 2 molecules condensing on CO2 nuclei causes the average N2 cluster size to increase from 10 (the homogeneous condensation result) to about 2,000. The predicted Rayleigh scattering intensity from the simulation results was found to agree well the AEDC experimental data.