This study describes a new optical sensor, the CREC-GS-Optiprobe. This sensor can be used In gas-solid reactors for characterizing clustering phenomena with minimum intrusion effects. Theoretical considerations allow to anticipate a narrow focal region at 4.5 mm away from the sensor tip. These theoretical predictions are confirmed with calibrations and experimental testings of the novel sensor. It is the goal of the present study to offer evidence, using this novel CREC-GS-Optiprobe, of cluster formation in down flow reactors. It is demonstrated through experimental studies that in down flow systems (d(p) = 65 mu m FCC particles, solid fluxes = 3-7 kg m(-2) s) that catalyst evolves, as strings of particles, with a relatively large size distribution: average cluster sizes are comprised between 2 and 6 d(p) with most of these strings having 3.5 rf,. Using theoretical considerations and solving the equations of motion, it is proven that hydrodynamic interactions between particles lead, in down flow reactors, to formation of particle agglomerates (clusters). Key parameters in this process are particle approaching times and particle approaching velocities. It is shown that particle agglomeration is a very rapid process taking place in downer units in a time scale much smaller than the few seconds expected residence times.