The formation of gaseous alkali sulfates is known to yield aerosols and may also contribute to deposition and corrosion in the combustion of solid fuels such as biomass. In the present work a model for the gaseous sulfation of alkali hydroxide (AOH) and alkali chloride (ACl) is developed. It relies on a detailed chemical kinetic model for the high-temperature gas-phase interaction between alkali metals, the O/H radical pool, and chlorine/sulfur species. Thermodynamic properties of a number of potassium and sodium species have been estimated from Gaussian 3 ab initio calculations. Particular attention is paid to alkali hydrogen sulfates and alkali oxysulfur chlorides as potential gas-phase precursors of A₂SO₄. Rate constants have been drawn from the literature, from analogy with known reactions, or from QRRK theory. A detailed reaction mechanism for sulfation is proposed. The alkali transformations proceed by a number of molecule–molecule reactions, which can be expected to exhibit ionic behavior. The sulfation is initiated by oxidation of SO₂ to SO₃. Sulfur trioxide subsequently recombines rapidly with AOH or ACl to form an alkali hydrogen sulfate, AHSO₄, or an alkali oxysulfur chloride, ASO₃Cl. According to the present work, both of these complexes are sufficiently stable in the gas phase to act as precursors for alkali sulfate. Alkali hydrogen sulfate and alkali oxysulfur chloride are subsequently converted to alkali sulfate in fast shuffle reactions such as ASO₃Cl + H₂O→AHSO₄ + HCl and AHSO₄+ACl→A₂SO₄ + HCl. Modeling predictions compare favorably with the experimental results of K. Iisa et al. [Energy Fuels 13 (1999) 1184–1190] on the gas-phase sulfation of potassium chloride at 1373 K. They investigated the degree of sulfation of KCl in a gaseous mixture of SO₂ + O₂ + H₂O + N₂ as a function of reaction time and gas composition in an entrained flow reactor. The modeling predictions are not sensitive to the estimated properties in the alkali subset (thermodynamic data, rate constants) within the assigned error limits; the predicted degree of sulfation is influenced mainly by the rate of oxidation of SO₂ and the production of chain carriers in the system.