In order to reduce the pollutant emissions (NOx and PM) of diesel engines, the addition of small gaseous fuel amounts or dual mode operation have been proved as potential techniques. This paper is focused on a detailed characterization of the particles emitted from a single cylinder diesel engine when part of the diesel fuel (5 to 20% by energy) is replaced by a gaseous fuel (producer gas, mainly composed by H2, CO, CH4 and inert compounds) coming from biomass steam gasification. The engine was run at constant speed and torque and different EGR rates. Particle samples were collected by means of fiber glass filters placed in a dilution mini-tunnel. Simultaneously, during tests, part of the exhaust gas was conducted to an SMPS to determine the particle size distribution. The filters were analyzed by using two different techniques: Thermogravimetric analysis (TGA) to obtain the volatile organic fraction (VOF) and the soot concentration, and Soxhlet extraction/High Performances Liquid Cromatography (HPLC) to speciate PAHs (Polycyclic Aromatic Hydrocarbons).Results show a significant decrease in the PM mass and concentration when increasing the gaseous fuel addition. Such reduction is caused by the replacement of a carbon-rich fuel with a high potential to produce particles (diesel fuel) and by chemical effects (higher OH concentration due to the H2 presence, which promotes soot oxidation). Moreover, the addition of producer gas allows higher EGR rates (to reduce NOx) while keeping low PM emissions. The PM volatile fraction was increased for higher diesel fuel replacements. The PAHs content was very low (the lighter ones being the most significant) and it decreases when replacing diesel fuel by producer gas.