We have studied for the first time the photophysics of the thioketone, 4H-1-benzopyrane-4-thione (BPT), in water and deuterated water, by using femtosecond fluorescence up-conversion and transient absorption spectroscopies. We propose that the S-2-state deactivation occurs in the hydrogen-bonded BPT-water complex. The lifetime of the complex in the S-2 state is found to be as short as 1.0 +/- 0.1 ps in H2O, and 2.2 +/- 0.1 ps in D2O. (C) 2004 Elsevier B.V. All rights reserved.