Radiative lifetimes of vibrationally excited H2O+ and D2O+ ions in their ground electronic state (X B-2(1)) have been determined using the monitor ion technique in a triple cell ion cyclotron resonance spectrometer with Fourier transform detection. The monitor reactions are proton or deuteron transfer from H(DO+ to CO2 and N2O. The lifetimes are corrected for collisional deactivation and reactions with the background gases occurring during the relaxation time of the ions. N2O probes all the excited vibrational levels of H2O+ and D2O+. For H2O+ only th e bending modes (O, v greater than or equal to 1,0) contribute to the decay curve. The corresponding overall lifetime, 26.8 +/- 3 ms, is in very good agreement with the computer simulated overall lifetime including the theoretical lifetimes of Weis et al. [J. Chem. Phys. 91, 2818 (1989)] and estimated populations of the bending vibrational levels. For D2O+, the overall lifetime of the (O, v greater than or equal to 1,0) bending modes, 99.5 +/- 15 ms, and the lifetime of the (1,0,0) stretching mode, 27.5 +/- 4.5 ms, are observed, also in good agreement with the computer simulated and theoretical values, respectively. For both ions the overall lifetime of the (O, v greater than or equal to 1,0) levels may be considered as a good approximation for the radiative lifetime of the (0,1,0) level. The overall lifetimes determined with CO2 as a monitor may be attributed to the (O, v greater than or equal to 4,0) bending modes : 8.1 +/- 1 ms for H2O+ and 44 +/- 12 ms for D2O+. In this case, several levels having similar populations and lifetimes contribute to the decay curve, therefore the lifetimes of the individual levels cannot be determined. The agreement with computer simulated lifetimes is an indication for the validity of the theoretical lifetimes.