Lateral and axial profiles of solids cross-flow nux were measured in a circulating fluidized bed riser of square cross-section with a sampling probe. The solids cross-now flux was found to be higher near the wall, especially near the corners, and lower in the core of the riser. The net solids horizontal flux was outwards in the lower part of the riser and inwards near the top. The lateral solids momentum flux, measured by means of a piezoelectric probe, increased with height and then decreased near the top of the riser. The lateral solids momentum flux first increased with distance from the axis and then decreased towards the wall. It was especially low near the corners.