The formation of aqueous ammonium chloride aerosols has been investigated both experimentally and by simulation. The considered process is a flue gas cleaning plant in which hot gases containing gaseous ammonia and hydrogen chloride at 200 degreesC enter a quench cooler in which they are brought in intense contact with water and chilled down. The processes of absorption, cooling and nucleation occur simultaneously. The simulations are based on a description of the process according to the film model combined with homogeneous nucleation theory. The experiments were done at a semi-technical flue gas cleaning plant. It is found that the proposed model is able to predict the qualitative behaviour of the system, especially its sensitivity to a change of the concentrations of the two pollutants at its entry. However, to get quantitative coincidence an adjustment is necessary. In this, the governing parameter is the surface tension of ammonium chloride. (C) 2003 Elsevier Science B.V. All rights reserved.