A five-fibre optical particle velocity measuring system was used to determine vertical velocities and the fractions of rising and falling particles in a circulating fluidized bed riser of 146 mm x 146 mm square cross-section. The superficial gas velocity and solids circulation rate are shown to affect lateral and axial profiles of particle velocity and the fraction of particles moving upwards. The thickness of the outer annulus downflow region first decreased with height until a minimum was reached and then became thicker towards the top of the riser. Because of the exit effect, lateral profiles of particle velocity were asymmetric at the top of the riser and the wall downflow layer was thicker on the side opposite to the exit than on the exit side. The corners were also found to be regions of greater downflow.