In this paper, we present the results of a series of experiments to determine the exhaust particulate size distributions from a number of diesel, gasoline and compressed natural gas (CNG) fuelled vehicles.The results show that all three types of vehicle produce significant populations of particulates under certain operating conditions. Particulates produced by gasoline and CNG engines tend to be smaller than for diesel engines. At low loads, there is a significant particulate distribution for diesel engines but much lower particulate numbers for both gasoline and CNG vehicles. Under these conditions, the gasoline particulate distribution has little structure but the CNG distribution is clearly bimodal. At higher loads, the number of particulates produced by diesel vehicles increases by an order of magnitude from idle and both the CNG and gasoline distributions are comparable in peak height. The diesel vehicle produces a much larger particulate volume than gasoline or CNG.