Particle cluster velocities, cluster sizes, and solid holdups were measured in a cold-flow downer unit using CREC-GS-Optiprobes. Runs were developed in a 0.051 m ID and a 2 m high acrylic column and a feeding section with a cyclone and an eight-nozzle ring gas injector. A fluid catalytic cracking catalyst with a mean diameter of 84.4 mu m and a density of 1722 kg/m(3) was used. Measurements were obtained at 0.20, 0.40, and 1.80 m axial positions from the feeding section. Experiments were performed with a superficial gas velocity of 1.0-1.6 m/s and a solid mass flux of 30-50 kg/m(2) s. Measurements showed particle cluster distributions, which were close to normal in the feeding section and asymmetric in the stabilized region. Significant changes were noticed when clusters evolved from the feeding section to the stabilized section: cluster slip velocity values of 0.50.9 m/s in the downer entrance increased to 1.1-1.4 m/s in the stabilized region.