Brownian coagulation of aerosols in the transition regime was studied at Knudsen numbers ranging from 0.8 to 5.5. Coagulation rates were obtained from the change in total number concentration of polydisperse oleic acid and the sodium chloride aerosols with geometric mean diameters between 25 and 150 nm in a closed chamber at atmospheric pressure. Particle size distributions and concentrations were measured by scanning mobility particle sizer (TSI) and condensation nuclei counter (TSI), respectively. The experimental coagulation rates took deposition rates (diffusional wall loss rates) into account, because coagulation and diffusion deposition occur simultaneously in a closed chamber. In addition, to obtain the accurate coagulation rates of the experiments, two approaches were used and compared with each other. The experimental results were observed to follow the theoretical results of Dahneke and Fuchs, which showed that coagulation coefficients increased just a little in the transition regime. (C) 2003 Elsevier Ltd. All rights reserved.