This work evaluates the behavior of mercury in an industrial circulating fluidized-bed combustion plant (CFBC) of 50 MW situated in Oviedo, Spain. The results showed that mercury was not emitted in the gas phase in significant amounts, thereby proving this technology as an effective means of preventing mercury emissions. Most of the mercury (85-97%) was retained in the fly ashes. The proportion of mercury emitted in the gas phase was on the order of 0.2-0.3 mu g m(-3), whereas only 0.02 ng m(-3) at most was emitted in the particulate matter. An evaluation of the fly ash fractions separated from the hoppers of the electrostatic precipitator showed the highest proportion of mercury to be present in the fly ashes with the lowest particle size and the highest carbon content. However, this mercury proved to be stable when subjected to leachability tests.