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The excited triplet states of xanthone in organic glasses are studied by using a time-resolved ESR method at 77 K. The observed spectra are found to be superpositions of three kinds of triplet states in polar media. The mixing ratio of the three spectra varies with the change of the solvent. The spectrum that shows a small zero-field splitting parameter D becomes predominant in water-containing solvents and is assigned to a purely isolated 3 * state influenced by a strong hydration due to water molecule impurity. The other two spectra are assigned to the 3irx* states having large D values, which are strongly coupled through spin-orbit interaction with the 3 * state located a little higher in energy. The existence of these two 3inr* states in dry polar glasses may be attributed to structurally different conformers in the inhomogeneous circumstance of the glassy condition. In nonpolar glasses, no excited triplet state of xanthone is observed when an X-band ESR spectrometer is used because of its huge D value, but a weak total emissive signal of the excited 3 * of hydrated xanthone existing as an impurity is detected.

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