The behaviour of spherical solid particles in a horizontal channel flow is analysed using numerical calculations based on the Lagrangian approach. Recent developments in modelling particle motion, wall collisions, wall roughness, and inter-particle collisions are accounted for. The wall roughness model relies on the assumption that the impact angle is composed of the particle trajectory angle and a stochastic component due to wall roughness. A stochastic approach is used to describe inter-particle collisions between the considered particle and a fictitious collision partner which is a representative of the local particle phase. Then the collision probability is calculated on the basis of kinetic theory of gases, but accounting for the velocity correlation of colliding particles in turbulent flows. In order to allow an analysis of wall collisions and inter-particle collisions independent of the effect of particles on the flow, two-way coupling was neglected and flow and turbulence were prescribed. The particle behaviour for different boundary conditions, such as particle size, wall roughness and mass loading is discussed in detail. It is demonstrated that both effects, i.e. wall roughness and inter-particle collisions have a dramatic influence on the particle behaviour in a horizontal channel and the particle phase properties of the developed flow. In order to characterise the particle behaviour, the mean free path between wall collisions is introduced. In a second part of this work integral properties are presented in order to reveal the effects of wall roughness and interparticle collisions. Moreover, the effect of channel height on the particle phase properties is analysed. Finally, the calculations are compared with detailed measurements by phase-Doppler anemometry in a horizontal channel with a height of 35 mm and a length of 6 m for validation. (C) 2003 Published by Elsevier Science Ltd.