Although the variation of aerosol optical properties with humidity depends critically on particle size and chemical composition, existing models usually employ empirical growth factors for water uptake. For some size ranges and many types of particles these empirical factors are not well known. We describe here a model that applies a thermodynamic analysis to arbitrary aerosol chemical size distributions to compute water uptake, refractive index, number distribution, and optical extinction. By starting with the fundamental chemistry of the aerosols the model is able to compute the derivative of extinction with sulfate, sulfate plus ammonium, and other scenarios that may be derived from global change and pollution control strategies. It can also supply information on the variation of aerosol size and extinction with humidity and with details of the aerosol size distribution. We demonstrate this model using five detailed chemical size distributions measured with a cascade impactor in the marine boundary layer. The results are in good agreement with published extinction coefficients and with nephelometer data collected in concert with one of our samples. Calculated non-sea-salt sulfate mass-scattering efficiencies of our samples ranged from 0.6 to 2.6 g m-2 for dry particles and 5.3 to 13 g m-2 at 80% relative humidity. ¿ 1998 American Geophysical Union