The S-1 (1)A(2)(npi*) and S-2 (1)A(1)(pipi*) states of xanthone are phosphorescent due to strong coupling with the triplet state. To investigate the mechanism of intersystem crossing, we observed the phosphorescence excitation spectrum of xanthone in a supersonic jet. The S, (1)A(2)(npi*) <-- S-0 transition shows a well-resolved structure with sharp vibronic bands. Prominent bands are assigned to out-of-plane vibrational bands, of which the intensities arise from the vibronic interaction with the S-2 (1)A(1)(pipi) state. The vibronic bands of the S-2 (1)A(1)-(pipi*) <-- S-0 transition are significantly broadened (55 cm(-1)) and the band shape is Lorentzian. It indicates that the intersystem crossing is very fast from the S2 (1)A(1)(pipi*) state. The rate of intersystem crossing from the S-2 (1)A(1)(pipi*) state is estimated to be 1.0 x 10(13) s(-1).