Gas-solid fluidized-bed reactors operating in either conventional or circulating (riser or downer) modes are used in many applications such as fluid catalytic cracking and fluid coking. In such applications, uniform contact of the liquid feed droplets and entrained particles is essential for minimizing heat and mass transfer resistances and generating high yields of desirable products. This contact may be intensified in the mixing chamber of downer reactors. The objective of this study was to measure the quality of mixing between injected liquid feed and circulating solid particles in a downer mixing chamber. An efficient method was developed to determine the local quality of solid-liquid mixing oil a short time scale. The measurement technique used two types of measurements in order to obtain the cross-sectional distribution of liquid-to-solid ratios. First, temperature measurements were used to characterize the solid/liquid distribution and, second, tribo-electric probes were used to obtain the local solids fluxes. The measurement technique has proven to be a very reliable and reproducible method to determine the extent of liquid and solid mixing resulting from a variety of injection configurations. Very good and rapid contact between sprayed droplets and particles can be achieved by using appropriate mixing chamber geometries where four or more liquid spray jets impact each other. (c) 2005 Elsevier B.V. All rights reserved.