Gas-solid fluidized bed reactors have many industrial applications and have been studied extensively. Yet despite the amount of research on the subject there is considerable disagreement to their behaviour. Part of the disagreement is due to the varied techniques used to examine them. In this study we are examining, using non-intrusive techniques, the hydrodynamics of a partially scaled down reactor used in the production of polyethylene. In particular, we are using digital fluoroscopy to determine the properties of bubbles that form in the reactor. The goal of the research is to develop a tool that can be used to validate CFD models. The first part of the paper examines the tools used and software developed to study the properties of the bubbles. The second part of the paper discusses results from experiments conducted using glass beads (150 to 250 gm) and polyethylene particles (mean diameter of 850 lint and density of 650 kg/m(3) with a wide particle size distribution), which were used to validate the developed experimental technique and software using correlations in literature. (C) 2004 Elsevier B.V. All rights reserved.