The very sensitive cavity ring-down method has been applied to the measurement of the absorption of 4-H-1-benzo-
pyrane-4-thione (BPT).in a supersonic jet in the wavelength region of the S0->S1 transition between 15800 and 16650 cm-1. The absorption energies of vibronic states corroborate previous assignments in the multi-photon excitation
and S0->T1phosphorescence excitation spectra of jet-cooled BPT. The symmetry forbidden 0,0 transition, S0,0->S1,0at
16522 cm-1, was found to be 7.5 times weaker than the absorption transition to the triplet origin, S0,0->T1z,0.