Solubilities of argon were determined in acetone + water mixed solvent over the full range of composition using a static method. The gas solubilities were measured at three temperatures of 288.15, 298.15 and 308.15 K. The solubility dependence on the temperature in the mixed solvent was measured and discussed, and the similarity between excess quantities of mixed solvent and those of gas solubilities was examined. The order of gas solubility of argon in acetone + water mixed solvent at the three different temperatures was 288.15 > 298.15 > 308.15 K in the composition range of 0 less-than-or-equal-to x2 less-than-or-equal-to 0.15. However, the reverse relationship with temperature was observed in the range of composition of 0. 15 < x2 less-than-or-equal-to 1.0. For all solubility curves, maximum and minimum values of solubility were observed at lower temperatures, but not at higher temperatures. Furthermore, the excess Ostwald coefficients on the basis of volume fraction which express the non-ideality of gas solubility in non-ideal solutions were defined, and they were expressed by a polynomial equation of the Redlich-Kister (1948) type.