Combustion aerosols were measured in a 22MW ( thermal energy) municipal waste incinerator. Different types of waste fractions were added to a base- load waste and the effect on aerosol formation was measured. The waste fractions applied were: PVC plastic, pressure- impregnated wood, shoes, salt ( NaCl), batteries, and automotive shredder waste. Also, runs with different changes in the operational conditions of the incinerator were made. Mass- based particle size distributions were measured using a cascade impactor and the number- based size distributions were measured using a Scanning Mobility Particle Sizer. The plant is equipped with flue gas cleaning and the penetration through this was determined. The particle morphology was investigated by Transmission Electron Microscopy ( TEM) and chemical analysis of the aerosol particles was made by Energy Dispersive X- ray Spectroscopy ( EDS). The mass- based particle size distribution was bimodal with a fine mode peak around 0.4 mm and a coarse mode peak around 100 mu m. The addition of NaCl, shredder waste, and impregnated wood increased the mass concentration of fine particles ( aerodynamic diameter below 2.5 mu m). In general the mass concentration was stable and close to the reference PM2.5- value of 252 +/- 21mg= m(3) ( std. T, P). The total number concentration deviated during runs and between runs spanning from 43.10(6) to 87 . 10(6)#/cm(3)( std. T, P). The aerosols formed were mixtures of dense and aggregated particles in all tests. The fine particles are mainly composed by alkali salts, zinc, and lead. The heavy metals Cu, Cd, Hg, and Pb are significantly enriched in the fine particles.