A new process for the production of superdense anhydrous soda ash uses a mixture of water and a high-boiling second solvent to lower the transition point at which anhydrous (Na2CO3) and monohydrous sodium (Na2O3 . H2O) carbonate are in equilibrium to below the atmospheric boiling point. The stable conditions for anhydrate were first established by measuring the water activity in saturated mixtures of water and ethylene glycol. With the results, fed-batch evaporative crystallization experiments were carried out. Both the water activity and the crystallization measurements showed that anhydrous soda was stable in boiling mixtures containing more than 22.5 wt. % ethylene glycol (on a salt-free basis). A subsequent continuous evaporative crystallization experiment produced anhydrous soda ash with a bulk density of 1,550 kg/m(3), which is substantially higher than that of any other atmospherically crystallized soda.