As automobiles passed a measuring point, we recorded the concentrations of carbon dioxide and aerosol black carbon (BC) in their dispersing exhaust plumes. After subtraction of background levels, the ratio of the increments of these species allows us to calculate the emission factor of BC per unit mass of fuel from each individual vehicle. These factors spanned a range of greater than two orders of magnitude, representing the emission of from 4 X 10-6 to 10-3 grams of aerosol black carbon per gram of carbon consumed In the fuel. Their distribution showed that 20 percent of the vehicles accounted for 65 percent of the emissions. The real-time measurement methodology allows for a determination of the distribution of emission factors across the actual population of sources. These results are similar to the wide range of carbon monoxide emission factors reported recently. © 1990 Air & Waste Management Association.