The effects of particle inertia and particle-to-fluid density on the time scales that measure the dispersion of particles by isotropic turbulence are investigated. Numerical calculations are performed by means of a kinematic simulation method coupled with Lagrangian tracking of discrete particles. Moreover, a simple semi-empirical model for predicting the time scales of the fluid velocity seen by a particle as well as of the fluid-particle velocity covariance and the particle velocity variance is proposed. Comparisons between simulations and model predictions are presented. (c) 2006 Elsevier Ltd. All rights reserved.