Gaseous and particulate emission rates from seven class 2B, one class 5 and six class 6 heavy-duty gasoline- and diesel-powered trucks were determined using transient chassis dynamometer test procedures. All vehicles were tested at approximately 70% of their rated gross vehicle weight over the Heavy-Duty Transient Cycle and the Durham Road Route driving cycles. The sensitivity of emission rates to vehicle configuration, engine design, and driving cycle characteristics was examined. Emissions characterization included total hydrocarbons, carbon monoxide, oxides of nitrogen, fuel economy, total particulate matter, CH2Cl2 %-extractables, particulate organics, inert material, particle size less than 2μ, and lead, bromine, and chlorine analyses. For a particular type of engine (gasoline or diesel), class 2B truck emission rates were less than class 5 or 6 truck emission rates. Hydrocarbon and carbon monoxide emissions and fuel consumption were significantly higher with the gasoline trucks than with the diesel trucks. Total particulate, particulate organics, and inert material emissions were significantly greater with the diesel trucks. Hydrocarbons, carbon monoxide, total particulate emissions, and fuel consumption were sensitive to the characteristics of the transient driving cycles.