In a 10 in tall and 76 mm 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 550 kg/m(2)s and superficial gas velocity up to 10 m/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 (< 200 kg/m(2)s), the radial profiles of particle velocity are significantly less uniform at high-flux operations (> 300 kg/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.