The exhaust soot particles number size distributions obtained from the combustion of diesel and RME fuels were investigated in a high speed direct injection (HSDI) diesel engine for different engine operating conditions viz., fuel injection pressure, injection timing, exhaust gas recirculation (EGR) and engine load. An Electrostatic Mobility Spectrometer (EMS) was used for characterising the exhaust soot particle number size distribution. Increasing the fuel injection pressure reduced the particle size and its number concentration in the accumulation mode under low and high load conditions, but an opposite trend was observed to the particle number concentration in the nucleation mode under higher load operation. The effect of fuel injection timings on the particle number concentration was not clear and consistent between diesel and RME fuels under low load operation. Under high load operation, the overall particle number concentration for RME decreased but for diesel only the nucleation mode decreased, while the accumulation mode remained unaltered when the fuel injection timing was retarded. The addition of EGR caused the particles to agglomerate and form larger size particles, which were observed mostly in the accumulation mode. Under most of the engine operating conditions RME emitted lower soot particle concentration than diesel under both nucleation and accumulation modes. The presence of oxygen in the fuel has the potential to lower the exhaust particle number concentration in diesel engines. (C) 2013 Elsevier Ltd. All rights reserved.