The slurry transformation of sodium dehydroacetate monohydrate with different compositions of water and methanol was investigated. The water content and temperature did not only affect the solubility but also the crystal form of the precipitated material. A new needle hydrate form of sodium dehydroacetate appeared on the surface of the anhydrate when the water content was between 2% and the critical transformation values which varied with temperature. The results revealed that the needle hydrate was a new form of sodium dehydroacetate monohydrate. A possible explanation for these observations was found in the relationship between the change of water content, the interaction between solute and solvent, and the influence of supersaturation.