Flow structures were determined in a circulating fluidized bed (CFB) riser (0.203 m i.d. x 5.9 m high) of FCC particles (d(p)=70 mum, rho(s)= 1700 kg/m(3)). A momentum probe was used to measure radial momentum flux profiles at several levels and to distinguish between upward and downward flow regions. Time-mean dynamic pressure (DeltaP(m)) decreases towards the wall in the range U-g=5-8 m/s, G(s)=10-340 kg/m(2) s. The thickness of the annular downflow layer based on DeltaP(m)=0 reaches a maximum with increasing height. The annular downflow layer disappears locally with increasing solids mass flux (G(s)) at a constant gas velocity, with achievement of the dense suspension upflow (DSU) regime. A new correlation is developed to predict the time-mean thickness of solids down-flowing layer based on solids mass flux and momentum flux. It successfully accounts for the variation of the annular layer thickness with height and Gs, and covers a wide Gs range right up to near the onset of the DSU regime. (C) 2004 Elsevier B.V. All rights reserved.