Particle dispersion in a square duct flow, is studied using large eddy simulation combined with Lagrangian particle tracking under conditions of one-way coupling. The flow has a bulk Re = 250 k, with six particle sizes ranging from 5 to 1000 mu m. Results obtained for the fluid phase show good agreement with experimental data. For particles, predictions demonstrate that secondary flows within the duct dominate small particle dispersion and result in a uniform distribution, whereas gravity promotes the deposition of large particles on the duct floor. For the largest particles, the secondary flows contribute to particle concentration in corners on the duct floor, with these particles also clustering in low-velocity regions close to the floor. A detailed analysis of the influence of the flow on particle distribution is provided through consideration of the particle dispersion function, with the mechanisms of particle dispersion elucidated using a dynamical analysis. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1667-1679, 2009