The initial stage of particle formation during gas phase synthesis is characterized by a high concentration. of particles that undergo rapid coagulation. The surface area of the agglomerates is reduced by coalescence. In this study a model is presented that accounts, in addition to nucleation and coagulation, for the shape of the particles by tracing both the particle volume and the total surface area density of the agglomerates. A moment model, based on a unimodal lognormal size distribution gives the concentration, polydispersity, and average volume of the particles as a function of space and time. The total surface area density of the particles is determined by solving an additional transport equation, incorporating a linear decay law to describe the decrease in surface area of a coalescing structure. The aerosol dynamics are calculated in combination with the convective and diffusive particle transport in a spatially inhomogeneous aerosol reactor. (C) 2003 Elsevier Science Ltd. All rights reserved.