The conditioning of aerosol particles to a prede ned composition and size can be considered as a standard problem in aerosol technology. Quite often aerosols generated by dispersing diluted solutions are conditioned in a subsequent ow reactor. To make the design of such reactors easier, a computer model was developed to simulate the behavior of particles with a volatile component during their passage through the reactor. The model is based on the assumption that part of the surface of the reactor is covered with a layer of the requested activity of the volatile component whose fraction has to be adjusted. The diffusion to or from this surface and the corresponding change of the particle size and composition is calculated for each streamline of the ow on the basis of a laminar ow pro le. At the moment data for the system H 2 O/H 2 SO 4 are implemented in the model, but an extension to other systems can easily be done. Circular as well as annular ow cross sections can be taken into account.