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Previously reported experiments revealed the presence of a small number of clusters or very small particles in the effluent of a nonthermal plasma reactor when treating a simulated engine exhaust mixture. These clusters are smaller than 7 nm. The quantity of clusters is orders of magnitude smaller than the particulate diesel or gasoline engine exhaust typically contains. In this report, we describe further experiments designed to determine the chemical composition of the clusters. Clusters were collected on the surface of a silicon substrate by exposing it to the effluent flow for extended time periods. The resulting deposits were analyzed by high mass resolution SIMS and by XPS. The SIMS analysis reveals NH4+, CH6N+, SO-, SO2-, SO3- and HSO4- ions. XPS reveals the presence of N and S at binding energies consistent with that of ammonium sulfate.