A one-stage dilution tunnel has been developed to sample and dilute diesel exhaust. The tunnel has the capability of simulating many aspects of the atmospheric dilution process. The dilution rate and overall dilution ratio, temperature, relative humidity, and residence time in the tunnel, as well as residence time and temperature in the transfer line between the tunnel and exhaust sampling point may be varied. In this work we studied the influence of the exhaust transfer line, tunnel residence time, and dilution air temperature on the exhaust particle size distribution. The influences of fuel sulfur content on the size distribution and on the sensitivity of the size distribution to dilution and sampling conditions were also examined. We do not suggest an optimum dilution scheme, but do identify critical variables. The final choice of dilution schemes must be informed by comparisons between particle measurements made in the plumes of vehicles in the atmosphere and those made in the laboratory. Such work is currently underway in the CRC E-43 program and will be reported in future publications. A long residence time in the transfer line from the engine to tunnel was shown to decrease formation of nuclei mode particles which were mainly in the 8 to 30 nm diameter range. The likely explanation is that particle precursors like sulfuric acid and hydrocarbons are adsorbed onto existing particles and the walls of the transfer line. Increasing residence time in the tunnel itself led to a modest increase in the number particles measured in the nuclei mode even at very high dilution ratios. Decreasing dilution air temperature markedly increased formation of nuclei mode particles. Neither tunnel residence time nor temperature influenced the concentration of particles in the accumulation mode (roughly the 30 to 300 nm diameter range). Increasing the fuel sulfur content increased the formation of nuclei mode particles but did not significantly influence the accumulation mode.