The influence of riser diameter on the axial and radial particle velocity profiles and flow development is studied in a 10 m high twin-riser system (76- and 203-mm ID risers). Cross-sectional average particle velocity is somewhat lower for the larger riser with a steeper radial particle velocity profile. The flow development in the riser center is nearly instant with the particle velocity remaining high. There is no significant difference for the two risers in the center region. In the wall region, the flow development is significantly slower and the particle velocity of the smaller riser is higher. The flow development slows down in the larger riser. In all locations measured, there was a clear dependency between the local particle velocity and solids concentration of both risers. Gas distribution and particle aggregation are considered the key factors that influence the local hydrodynamics in the twin-riser system. (c) 2005 American Institute of Chemical Engineers.