A theory for the turbulent diffusion of particles suspended in an incompressible fluid is presented. Special attention is paid to the influence on the turbulent diffusion of the two-way coupling between the carrier fluid and the dispersed particles. Also the influence of the hydrodynamic interaction between the particles and the influence of the finite-particle size are investigated. It is shown that the influence of the particles-fluid interaction and of the finite-particle size are significant. The hydrodynamic interaction between the particles appears to be negligible up to a volume fraction of 0.1. (C) 2002 Elsevier Science Ltd. All rights reserved.