A system has been developed that allows near real time measurements of total, volatile, and nonvolatile particle concentrations in engine exhaust. It consists of a short section of heated catalyst, a cooling coil, and an electrical aerosol analyzer. The performance of this catalytic stripper system has been characterized with nonvolatile (NaCl), volatile sulfate ((NH 4)2 SO4), and volatile hydrocarbon (engine oil) particles with diameters ranging from 0.05-0.5 μm. The operating temperature of 300°C gives essentially complete removal of volatile sulfate and hydrocarbon particles, but also leads to removal of 15-25% of solid particles. This system has been used to determine total, volatile, and nonvolatile particle concentrations in the exhaust of a Diesel engine and a spark ignition engine. Volatile volume fractions measured in Diesel exhaust with the catalytic stripper system increased from 19-65% as the equivalence ratio (load) decreased from 0.64-0.13. Volatile volume fractions agreed with volatile mass fractions measured by a filter vacuum sublimation method, except at low equivalence ratios (0.13-0.4) where the filter method gave somewhat higher values. Particles were measured in the exhaust of the spark ignition engine upstream of its catalytic converter. Concentrations are very low under normal operating conditions, in the same range as ambient air, but show a very strong increase with increasing equivalence ratio. This suggests that under rich conditions, e.g., cold starts, high altitudes, and high loads, particle emissions may be significant. Volume volatile fractions for the spark ignition exhaust particles are highest, about 60% at equivalence ratios near 1 and fall to less than 30% at an equivalence ratio of 1.35.