Size diffusion is often regarded as a problem encountered with the numerical solution of the general dynamic equation for aerosols. We investigate here the real size diffusion arising in the solution of the more fundamental discrete rate equations, which treat condensation and evaporation on a molecular basis. The spreading of the aerosol distribution in molecular number content is obtained from previously derived moment equations, and is verified by numerical calculations. When the condensing material is volatile and its supersaturation is low, spreading values are very significant in the nanometer size range. The spreading of the aerosol distribution in radius tends to a constant value at large sizes, but this corresponds to a fall-off in the geometric standard deviation of the size distribution in the logarithm of the radius. Experimental examination of distributions at small sizes has the potential to give information on the volatility of the condensing material. (C) 2004 Elsevier Ltd. All rights reserved.