Interparticle collision plays an important role in the mechanics of gas-solid two-phase flows. The paper presents direct measurements of collision rate as well as collision properties of spherical glass beads with sizes of 500 50 pm in the upper dilute zone of a cold pilot-scale CFB riser, by using a high-speed imaging system. The recording rate of the high-speed digital camera is as high as 5000 fps with a resolution of 640 x 480. A large number of particle movement images at a height of 3.54 m above the distributor plates were taken. Manual inspection and automatic methods based on digital image processing algorithms were carried out to analyze particle image sequences. The experimental results show that the measured particle collision rate is proportional both to the particles' average relative translational velocity and the square of the particle number density, which coincides with the collision theory derived according to the analogy of kinetic theory of gases. But the theoretical model is found to overestimate the real collision rates, and a coefficient a of 0.33 may be used to correct this discrepancy. The possible reasons for this discrepancy are also discussed. The measurement results of collision properties based on more than 50 particle collision events agrees well with Walton's hard-sphere collision model. The three collision parameters, i.e., the average coefficient of friction It, the normal and tangential coefficients of restitution e and beta(0). for the glass beads used are measured to be 0.175 +/- 0.005, 0.96 +/- 0.02, and 0.43 +/- 0.09, respectively. (C) 2008 Published by Elsevier Ltd.