A new method to evaluate horizontal dispersion of solids in bubbling fluidized beds is proposed: a carbon-loaded bed material prepared using the capacitance effect (volatile matter captured by porous particles) is used as a tracer. The tracer particles are identical to the remainder of the bed material, except for carbon-loading of a few weight percent. The extent of solid dispersion is evaluated by measuring the horizontal concentration profile of CO, which is formed by the reaction of carbon in the tracers with CO2 in the fluidizing gas. Transient change in the horizontal CO concentration profile is measured in the freeboard and the experimental results are compared with the theoretically calculated results of one-dimensional diffusion of solids. Thereby, the horizontal dispersion coefficient is determined. The experimental results agree with results obtained from the literature. (C) 2007 Elsevier B.V. All rights reserved.