The solubilities of l-glutamine in mixed solvents (water + ethanol, water + acetone) are a function of temperature, increasing with an increase in temperature. In the mixed solvent systems, the increase of R increases the solubility, which indicates that pure water has a high dissolving power. Among the two different mixed solvent systems, only if it has the same solvent molar ratio (R), the molar fraction solubility will decrease in the same degree. The ratio of the solubility in water to that in the mixed solvents of R = 20 or R = 40 almost keeps constant along with temperature. The dissolution enthalpy and entropy of l-glutamine in water predicted from the solubility data using van’t Hoff equation is less than that in mixed solvents.