A revised homogeneous ternary nucleation model in H2O-H2SO4-NH3 vapors is presented. The model is based on a self-consistent version of classical nucleation theory with a rigorous treatment of nucleation kinetics. The calculation of equilibrium vapor pressures is completely revised and the effect of H2O-H2SO4 hydration is considered in detail. Compared to earlier models, the new model is able to predict nucleation rates over a wider range of temperatures and trace gas concentrations. A considerable dependence on relative humidity is found. The critical clusters corresponding to significant nucleation rates typically contain less than ten molecules and consist almost exclusively of H2SO4 and NH3. (C) 2002 American Institute of Physics.