Analysis of the 1H‐17O dipolar contribution to the proton longitudinal relaxation rate in 17O‐enriched water yields a correlation time for re‐orientation of the O–H vector τrOH = (1.71 ± 0.07) ps in H2O at 25 °C. This result is insensitive to uncertainties in the calculated intermolecular relaxation contribution. From 1H, D, and 17O relaxation rates in H2O/D2O mixtures deuterium isotope effects on the correlation times were estimated. The quadrupole coupling constants of D and 17O in liquid water were determined.