Particle formation and emission in the combustion of four (orujillo, eucalyptus, oak and chestnut tree) pulverized biomass fuels have been studied. The fuels have been burned in an entrained flow reactor, under controlled and realistic conditions. In all the cases, the final emission distributions contained at least two modes, one of them peaking at 30-200 nm, and the other being in the supermicron range. Alkali sulphates and chlorides account for most of the mass of the fine particles in all cases, while coarse particles essentially retain the inorganic matter properties of the original fuel. K2SO4 is experimentally found to start nucleation over 900 degrees C for orujillo, while KCl is not observed at this temperature. Condensation of KCl on these nuclei is observed in a sample taken at 560 degrees C and in a greater amount at 360 degrees C. The same formation schema was found for the other biomass fuels; the much lower ash content of the latter is thought to cause a "delay" in the onset of these nucleation/condensation steps along the gas cooling process. In spite of these differences, the global similarities observed among the studied fuels confirm the generality of the fine particle formation process in biomass combustion.