Racemic tartaric acid (TA) exists in the solid state as an anhydrate and a monohydrate. In this paper, a method to obtain the anhydrate at lower temperature by adjusting the water activity, a(w), through introduction of ethanol is proposed. The influence of a, on the phase conversion temperature of TA was investigated by the determination of the ternary phase diagrams for the system of water-ethanol-TA at varying temperatures. The result shows that the anhydrous TA can be obtained at ambient temperature by lowering the water activity and that the conversion rate from monohydrate to anhydrate appears to increase with decreasing water activity. The result can be extended to the application of other anhydrate and hydrate systems. Meanwhile, the process of anhydrous TA crystal formation from ethanol-water solution was investigated by using microscopy and HPLC,