A colorless crystalline substance was produced from dulcin dissolved in water by boiling. This substance was proved to be different from dulcin as results of taste, melting point, UV absorption spectrum, IR absorption spectrum and elementary analysis, and it was regarded as didulcin. When the aqueous solution of dulcin was heated at different temperatures, dulcin was unstable at high temperature, however, it was not changed below 60°C within about one hour.