Experiments were undertaken to determine some of the characteristics of exhaust particulate emissions in two port fuel injected spark ignition engines. A 2.3L 1993 GM Quad-4 engine and a 4.6L 1994 Ford V8 were tested. Sampling and dilution were accomplished through the use of a single-stage, low residence-time ejector diluter; dilution ratios were maintained at approximately 15:1. Number concentration was measured with a TSI 3020 condensation nucleus counter, and size distributions were measured using two scanning mobility particle sizers.The Quad-4 engine was used to determine the effects of the catalytic converter and deposit control additives on particulate emissions. The catalyst was found to remove particles with an efficiency as high as 78% at low power conditions (∼7 kW), dropping steeply with power, reaching a minimum value of approximately 10% at moderate power conditions (∼18 kW). Particle size distributions showed a decrease in number concentration for particles smaller than 70 nm and larger than 200 nm.Of the two deposit control additives tested, one produced a significant decrease in the overall particle number emissions while another additive produced an increase in number emissions. These changes in number concentration were particularly evident at high power conditions.The Ford engine was tested to determine its general particulate emissions behavior in terms of number emissions. The Ford engine exhibited spiking behavior in terms of time-resolved particle number emissions. Average particle number concentration (as well as number-weighted size distributions) remained fairly constant over the range of engine operating conditions tested. The level of deposits present in the engine did not seem to have any effect on particulate emissions.