Particle-turbulence interaction for dilute gas-particle flows over a back-ward-facing step geometry is numerically investigated. An Eulerian two-fluid model with additional turbulence transport equations for particles is employed in this investigation. RNG based k-epsilon model is used as the turbulent closure with additional transport equations solved, to better represent the combined gas-particle interactions. Two different particle classes with same Stokes number and varied particle Reynolds number are considered in this study. The turbulence modulation of the carrier phase in the presence of the dispersed particulate phase is simulated and compared against the experimental data. However prior to this endeavour, the simulated flow field is validated for mean streamwise velocities and fluctuations for both the phases. Despite the fact that the two particles used in this study share the same Stokes number their behavior is found to be considerably different in the turbulent flow field, which basically underlines the fact that the Stokes number alone is not enough to fully describe the behavior of particles, thereby, herein particle Reynolds number is also investigated to fully understand their behavior. Two other turbulence modulation models were also tested against our own formulation, and our model was found to compare better with the experimental findings. (C) 2007 American Institute of Chemical Engineers.