A model is developed for the coalescence of unequal-sized amorphous primary particles commonly found in aerosol aggregates. Computational efficiency is achieved by deriving an analytical expression for the surface geometry of the coalescing particles and subsequently decoupling surface evolution from the fluid dynamic equations. Calculated shrinkage lengths and surface areas are in good agreement with results reported from finite element calculations for all particle size ratios considered. Results indicate that above a size ratio of 4, the normalized coalescence rate is independent of size ratio. Applying these results to the coalescence of SiO2 particles indicates that rates are primarily dependent on the diameter of the smaller particle, although coalescence times do not scale linearly with either particle size ratio or total particle mass. Comparison with the widely used exponential model of Koch and Friedlander suggests that rates of surface area decrease are under-predicted by the Koch and Friedlander model. Modified values of the decay constant for both equal sized and heterogeneously sized particles are therefore proposed, with the value decreasing with increasing size ratio of the coalescing pair. (C) 2003 Elsevier Ltd. All rights reserved.