We determined adsorption kinetics for a representative aromatic hydrocarbon, m-xylene, and three types of fly ash, two from commercial power plants and one from an experimental fluidized-bed combustor. Rate constants and adsorption half-times were compared to fly ash residence times in coal combustion power plant stacks. Kinetics of m-xylene adsorption by fly ash resembled kinetics reported for penetration of absorbates into porous adsorbents. No increase in adsorption rates was observed with increased temperature; rate constants decreased with increase in vapor pressure. This suggested that adsorption was diffusion limited. The residence times of fly ash in the stack were similar to the half-times required for monolayer formation, but were much shorter than the half-times characteristic of multilayer formation and pore filling.