One hundred three in-use vehicles identified as high hydrocarbon and/or carbon monoxide emitters by remote sensing and roadside visual, underhood checks during a 1995 South Coast Air Quality Management District program were tested on the IM240 cycle using a transportable dynamometer. Seventy-one of these vehicles were repaired as part of the program and were retested. Seventeen vehicles in the fleet initially emitted visible smoke from the tailpipe and were classified as “smokers”. The fleet ranged in age from 6 to 22 years, with a median age of 12.3 years. Exhaust HC, CO, NOx, and particulate emissions (PM-10) were measured. PM-10 mass and the elemental and carbonaceous composition of the particulate matter were determined. The average fleet PM-10 emission rate was 0.138 g/mi, while the average emission rate for smokers was 0.395 g/mi. It was concluded that the casual observation of smoking vehicles was not very successful in identifying high PM-10 emitting vehicles. The particulate matter composition was primarily carbonaceous, with a variable distribution between the elemental and organic carbon fractions, and showed no significant difference in the percent organic carbon fraction between smokers and non-smokers. The PM composition differed, on average, between pre-repair and post-repair samples. The effect of the California Smog Check repair program on PM emission rates could not be evaluated due to concerns regarding the effect of vehicle conditioning on these measurements.