In this study, the influence of CO2 on sulfur capture efficiency was studied during fluidized-bed desulfurization by experiments and modeling. During calcination–sulfation and direct sulfation, the effect was examined with one limestone type. A time-dependent multilayer particle model was used for analyzing the experimental results. The model determines the magnitude of the reactions and the diffusion as a function of the radius and time. In high temperatures (∼1200 K), CO2 increased the conversion degree during calcination–sulfation. In direct sulfation, the effect of CO2 was opposite; lower conversion was obtained when the CO2 concentration was increased. When the CO2 concentration was increased in low temperatures (∼1100 K) (close to the calcination curve), CO2 retarded the conversion strongly. The detected differences between the results are explained with the development of the Thiele number, conversion curve, and conversion profile during the reactions.