Thermodynamic calculations are used to study the alkali reactions in coal gasifier atmospheres. The reactive alkali and sulfur species released from coal are first calculated at temperatures from 800 K to 1900 K and pressures from 1 atm to 100 atm. Four P-T diagrams are constructed for the stable alkali and/or alkali-sulfur species at differ-ent temperatures and pressures. Alkali vapors are generated by the reactions Na₂CO₃_(s) + 2C_(s) = 2Na_(g) + 3CO_(g) Na₂CO₃_(s) + H₂O_(g) + C_(s) = 2NaOH_(g) + 2CO_(g) or K₂CO₃_(s) + 2C_(s) = 2K_(g) + 3CO_(g) K₂CO₃_(s) + H₂O_(g) + C_(s) = 2KOH_(g) + 2CO_(g) The phases formed from alkali-cement, and alkali-sulfur-cement reaction are also predicted. For both 53% and 72% alumina cement, calcium aluminate (CaO•Al₂O₃) is decomposed by the reactions CaO•Al₂O₃ + 2Na + 1/20₂ = Na₂O•Al₂O₃ + CaO CaO•Al₂O₃ + 2K + 1/20₂ = K₂O•Al₂O₃ + CaO or CaO•Al₂O₃ + 2Na + l/2S₂ = Na₂0•Al₂O₃ + CaS CaO•Al₂O₃ + 2K + 1/2S₂ = K₂•Al₂O₃ + CaS / M.S.