Exhaust particulate emissions from a 4-cylinder, 2.25 liter spark ignition engine were measured and characterized. A single-stage ejector-diluter system was used to dilute and cool the exhaust sample for measurement. The particulate measurement equipment included a condensation nucleus counter and a scanning mobility particle sizer. Exhaust measurements were made both upstream and downstream of the catalytic converter using three different fuels.Unlike particulate emissions in diesel engines, spark ignition exhaust particle emissions were found to be highly unstable. Typically, a stable “baseline” concentration on the order of 105 particles/cm3 is emitted. Occasionally, however, a “spike” in the exhaust particle concentration is observed. The exhaust particle concentrations observed during these spikes can increase by as much as two orders of magnitude over the baseline concentration. The spikes were found to be composed of nearly 100% volatile particles which were below 30 nm in diameter.Particle number emissions at a 120 km/hr equivalent operating condition were found to be on the order of 1011-1012 particles per kilometer, more than two orders of magnitude below emissions previously reported for SI engines. The brake specific particle number emissions ranged from 1.6 × 1011 particles per kilowatt-hour at light load conditions to 7.3 × 1013 particles per kilowatt-hour at high load conditions. Brake specific particle emissions at 2000 and 2500 rpm were found to increase exponentially as a function of engine load, while the number-weighted geometric mean diameter was found to increase linearly as a function of engine load. The type of fuel used did not affect general trends, though more than a factor of two variation in the magnitude of the exhaust emissions was observed between the fuels.