We present a study on the relaxation of the O¿H stretch vibration in a dilute HDO:D2O solution using femtosecond mid-infrared pump-probe spectroscopy. We performed one-color experiments in which the 01 vibrational transition is probed at different frequencies, and two-color experiments in which the 12 transition is probed. In the one-color experiments, it is observed that the relaxation is faster at the blue side than at the center of the absorption band. Furthermore, it is observed that the vibrational relaxation time T1 shows an anomalous temperature dependence and increases from 0.74 ± 0.01 ps at 298 K to 0.90 ± 0.02 ps at 363 K. These results indicate that the O¿HO hydrogen bond forms the dominant accepting mode in the vibrational relaxation of the O¿H stretch vibration. ©1999 American Institute of Physics.