Small amounts of enriched H217O and 3He in gaseous mixtures with CH3F and CF3H were studied using 1H, 3He and 17O NMR spectroscopy. After extrapolation of the results to the zero density limit, the shielding constants in the isolated molecules: H217O, H17OD and D217O were precisely determined. The isotope effects are as follows: 2Δ1H(HOH, HOD) = -0.040 ppm, 1Δ17O(H2O, HOD) = -1.51 ppm and 1Δ17O(HOD, D2O) = -1.48 ppm.