Direct numerical simulations of nanoparticle coagulation in temporal mixing layers using two different formulations for particle diffusion are compared. In the first formulation, a diffusion coefficient based on the local mean volume is used while the second formulation utilizes diffusion coefficients which are based on each particle size. A moment method in conjunction with a lognormal particle size distribution is used to represent the particle field. Results indicate that simulations using diffusion coefficients based on the local mean volume predict slightly higher polydispersity. Furthermore, the simulations using diffusion coefficients based on the local mean volume can be performed at reduced spatial resolutions which allows for more affordable computations. (C) 2003 Elsevier Ltd. All rights reserved.