SO3 emissions from coal-fired plants cause equipment corrosion and environmental pollution, which can be alleviated by adsorbent injection. The dynamic adsorption of SO3 by Na2SO3 was studied with a fixed-bed reactor at 250-350 degrees C under simulated flue gas. An obvious competitive reaction of Na2SO3 oxidation by O-2 was found during SO3 adsorption. The conversions of Na2SO3 by SO3 adsorption and by O-2 oxidation all rapidly increased in the initial stage and then slowly increased. The selectivity of SO3 adsorption by Na2SO3 also rapidly increased first and then slowly increased, except at 250 degrees C. It was between 0.06 and 0.12 in the later stage. Decreasing temperature and increasing SO3 concentration increased the SO3 selectivity. Na2SO3 oxidation was also promoted by the increasing SO3 concentration. Pore structure analysis showed that a more porous product layer was formed by the enhanced SO3 adsorption, facilitating the O-2 diffusion. As the product gas of SO3 adsorption, the presence of SO2 in the reactant gas suppressed the forward reaction of Na2SO3 and SO3. Thus, increasing SO2 concentrations weakened SO3 adsorption and decreased the SO3 selectivity. The strong competitive reactions greatly reduced the effective utilization of adsorbents, which should be considered during deSO(3) process design.