Rhodamine 6G is a xanthene dye with high fluoresce quantum yield, and hence it has been used for organic-dye laser and organic light-emitting diode. Rhodamine 6G is, however, very unstable for continuous light irradiation in acetone and water. We demonstrated that rhodamine 6G in acetone was stabilized by addition of t-butyl-4-hydroxyanisole (BHA) without decrease of fluoresce intensity. Rhodamine 6G in acetone was not stabilized by β-carotene. These results suggest that instability of rhodamine 6G in acetone was caused by not active oxygen but some radical species produced by light irradiation. Rhodamine 6G was found to be remarkably stable in acetone/water (1/1, v/v), where gem-diol was generated. The formation maximum of gem-diol was estimated to be performed when the ratio of acetone/water = 1/1 (v/v) from the viewpoint of equilibrium reaction. Therefore rhodamine 6G in acetone/water = 1/1 was stabilized by gem-diol. High stability of rhodamine 6G in ethanol and methanol also support the hypothesis.