In the present modeling work, the gas-phase alkali sulfation process is investigated. The combined effects of oxyfuel combustion and co-combustion on the sulfation of alkali metals are examined and compared to the corresponding process under air-fired conditions. According to the modeling results, the degree of sulfation of gas-phase alkali metals is generally higher for oxy-fuel combustion than for air fuel combustion. The recirculation strategy, in terms of the flue gas composition in oxy-fuel combustion, is important for the sulfation of alkali metals. The increased contents of sulfur and water contribute significantly to the degree of sulfation of the alkali species. The modeling also reveals that an increased content of HCl, which is present during wet recirculation, has a negative effect on the sulfation. Thus, the degree of sulfation is lower in wet recirculation than in dry recirculation. The effects of other important parameters, such as the sulfur/potassium ratio, oxygen concentration, and temperature, are discussed.