*v*= 1) and OD(

*v*= 1) by H

_{2}O and D

_{2}O as a function of temperature between 251 and 390 K. All four rate coefficients exhibit a negative dependence on temperature. In Arrhenius form, the rate coefficients for relaxation (in units of 10

^{−12}cm

^{3}molecule

^{−1}s

^{−1}) can be expressed as: for OH(

*v*= 1) + H

_{2}O between 263 and 390 K:

*k*= (2.4 ± 0.9) exp((460 ± 115)/

*T*); for OH(

*v*= 1) + D

_{2}O between 256 and 371 K:

*k*= (0.49 ± 0.16) exp((610 ± 90)/

*T*); for OD(

*v*= 1) + H

_{2}O between 251 and 371 K:

*k*= (0.92 ± 0.16) exp((485 ± 48)/

*T*); for OD(

*v*= 1) + D

_{2}O between 253 and 366 K:

*k*= (2.57 ± 0.09) exp((342 ± 10)/

*T*). Rate coefficients at (297 ± 1 K) are also reported for the relaxation of OH(

*v*= 2) by D

_{2}O and the relaxation of OD(

*v*= 2) by H

_{2}O and D

_{2}O. The results are discussed in terms of a mechanism involving the formation of hydrogen-bonded complexes in which intramolecular vibrational energy redistribution can occur at rates competitive with re-dissociation to the initial collision partners in their original vibrational states. New

*ab initio*calculations on the H

_{2}O–HO system have been performed which,

*inter alia*, yield vibrational frequencies for all four complexes: H

_{2}O–HO, D

_{2}O–HO, H

_{2}O–DO and D

_{2}O–DO. These data are then employed, adapting a formalism due to Troe (J. Troe,

*J. Chem. Phys.*, 1977, 66, 4758), in order to estimate the rates of intramolecular energy transfer from the OH (OD) vibration to other modes in the complexes in order to explain the measured relaxation rates—assuming that relaxation proceeds

*via*the hydrogen-bonded complexes.