Geometrical relationships for two unequal-sized coalescing aerosol particles in the entire coalescing process are obtained by considering the coalescing process at the point where the neck size equals the diameter of the small particle and under the assumption that each particle is in the shape of a spherical cap. Comparisons on the shrinkage length have been made between present model and the Yadha and Helble [(2004). Modeling the coalescence of heterogenous amorphous particles. Journal of Aerosol Science, 35, 665-681] geometrical model. Excess surface area of titania nanoparticles sintering at different temperatures has also been compared between predictions from the present model, the Koch and Friedlander [(1990). The effect of particle coalescence on the surface area of a coagulating aerosol. Journal of Colloid and Interface Science, 140, 419-427] model, and the literature data. The results show that the present model gives reasonable predictions based on an overall solid-state diffusion mechanism at the start and the volume solid-state diffusion mechanism at later stages of the coalescing process. (c) 2007 Elsevier Ltd. All rights reserved.