In a 10m tall and 76mm i.d. high-flux riser, radial profiles of particle velocities were determined with a five-fiber optic probe on seven axial levels, with solids circulation rate (solids flux) up to 550kg/m 2 s and superficial gas velocity up to 10m/s. This study shows a more comprehensive set of radial profiles under higher fluxes than any previous literature. Four axial stages of flow development were distinguished based on radial distributions of particle velocity and concentration. Compared to low-flux operations (<200kg/m 2 s), the radial profiles of particle velocity are significantly less uniform at high-flux operations (>300kg/m 2 s). The superficial gas velocity has more influence on the radial velocity profiles than the solids flux at high solids flux. Net solids downflow only occurs near the wall under limited conditions in the bottom section.