The effects of geometric and operating parameters and feed characters on the motion of solid particles in hydrocyclones were experimentally investigated by using a new type of laser surveying instrument named particle dynamics analyzer. The absolute radial velocity of solid particles decreases with increasing the positional radius, and the axial distribution curves of the particle radial velocity are parabolic. The particle radial velocity increases with increasing the inlet pressure or with increasing the diameter of the underflow pipe. When the particle density or the particle size increases, the absolute radial velocity of the solid particles decreases. The particle radial velocity also decreases with increasing the feed particle concentration. The axial distribution curves of the particle axial velocity are also parabolic. The axial velocity in the inner helical flow increases with the increase of the flow rate of overflow; while that in the lower positions in the outer helical flow increases with increasing the flow rate of underflow. (C) 2002 Elsevier Science B.V. All rights reserved.