Aerosol chargers, which deliver defined charge distributions, are needed for accurate studies of particle size distributions. However, the aerosol residence in the charger volume leads to changes in the particle size distribution mainly due to Brownian and electrical coagulation. The effect of coagulation is clearly shown by this study, where changes in the particle number concentration and geometric mean diameter were investigated for neutralizer and UV-charger in dependency on the initial particle number concentration and residence time. The process of simultaneous charging and coagulation is simulated by a specially developed Monte Carlo method. Experimental results show a good agreement with results of Monte Carlo simulations. It is demonstrated, that the coagulation significantly changes the parameters of the particle size distribution during residence in the chargers for particle number concentrations exceeding 10(13) 1/m(3). This result is important for accurate measurements of highly concentrated aerosols. (C) 2004 Elsevier Ltd. All rights reserved.