Particle clustering is a phenomenon of importance in many applications in gas-solid fluidized beds. Using a novel Chemical Reactor Engineering Centre Gas-Solid Optiprobe (CREC-GS-Optiprobe) equipped with a Graded Refractive INdex. (GRIN) lens to form a 285 gin diameter high illuminated region at 5.4 mm away from the tip of the sensor, particle cluster lengths were measured with minimum probe intrusion interference. Experiments were developed in a 3 in height and 0.0263 in diameter acrylic unit using a pair of CREC-GS-Optiprobes spaced 6.05 mm with the probes placed at 1.85 m away from the gas injection port. Operating conditions were varied from 1 to 3 m/s gas superficial velocities and from 2 to 12 kg/m(2) s particle solid fluxes resulting in solid hold-ups between the 0.001 and 0.003 ranges. Acquired signals displayed distinctive double peaks showing unequivocally the simultaneous detection of particle clusters on both probes. It is concluded that in down-flow units (FCC particles of 70 mum) catalyst particles evolve as particle strings with a relatively large size distribution: average cluster size ranging between 2.0 and 6.3 times of the average particle diameter with most of the particles strings having between 2.9 and 5.5 times of the average particle diameter. (C) 2004 Elsevier B.V. All rights reserved.