Measurements of particle flux are reported for air/particle flow in and near a diffuser in the riser of a circulating fluidised bed. A diffuser is here defined as a duct of tapered cross section, the larger cross-section at the top; the top and bottom are each connected to a vertical duct of uniform cross-section. In the present work, the top and bottom sections were square: the top section was 0.14 x 0.14 m; the bottom section was 0.11 x 0.11 m: the diffuser connecting the two sections was slab-sided, each side being inclined at 6.8 degrees to the vertical; the total duct height, comprising the diffuser and the two parallel sections, was 5.1 m. Go-current upflow of air and cracking catalyst, mean diameter 60 mu m, was studied. The conditions were chosen to give similarity with a large industrial circulating fluidised bed (CFB): the air velocity was 1.3-2.1 m/s and the flux net particle flux 2.3-3.8 kg/m(2)/s. Upward and downward particle flux profiles, across sections in the parallel ducts and at the top and bottom of the diffuser, were measured with a sampling probe 3.4 mm in diameter. Interpolation algorithms gave flux profiles across each section, showing core-annulus flow. integration of these profiles across the duct gave the net particle flow, in good agreement with external measurements using a slot flow meter. While single phase (air only) flow in the duct showed unseparated motion in the diffuser, the flux profiles for solids suggest a strong recirculation of solids and probably air also, in the diffuser. Adjacent to the wall is the usual region of solids downflow; the mean thickness of this region in the diffuser is about twice as much as in the parallel sections remote from the diffuser. Likewise the reflux ratio = (Particle downflow at a section)/(External particle circulation rate) is 2-3 times as much in the diffuser as compared with the parallel sections. For any industrial CFB including a diffuser, the results imply increased particle mixing in the diffuser, but the higher solids downflow, especially in the corners of a square section diffuser, may increase wall erosion.