In order to obtain fundamental data for alkali metal vaporization, the evaporation rate of alkali chlorides and alkali sulfates (NaCl, KCl, Na2SO4, K2SO4), which are major alkali compounds present in coal, were measured under inert, oxidizing and reducing atmosphere by using a thermobalance. A diffusion model was applied to obtain alkali metal evaporation data and identify an evaporating path and a rate controlling phenomena. Alkali chlorides evaporated as monomers and dimers. The evaporation rates ; explained by a diffusion model. In the case of alkali sulfates, a dissociation reaction occurred simultaneously with evaporation as monomer. In inert and oxidizing atmospheres, the dissociation reaction was represented as M2SO4 double right arrow 2M + SO2 + O-2 Evaporation rates can be calculated by a diffusion model in oxidizing atmosphere. In an inert atmosphere, however, when dissociation becames remarkable evaporation rate is controlled by the dissociation rate. Under a reducing atmosphere the expected reaction was represented as M2SO4 + 2CO double right arrow 2M + SO2 + 2CO(2) This reaction does not reach equilibrium, so that the following reactions control evaporation rate. M2SO4 + 4CO double right arrow 2M + COS + 5CO(2) COS + 2CO(2) double right arrow SO2 + 3CO.