High-precision triple oxygen isotope data on 40 water samples (5-10 923 m in depth) collected at the Challenger Deep of the Mariana Trench are reported in this study. The analyses at Laboratoire des Sciences du Climat et I'Environnement yield mean isotope values delta O-18 = -0.084 +/- 0.224 parts per thousand, delta O-17 - 0.061 +/- 0.117 parts per thousand, and O-17-excess = -17 +/- 5 per meg, with standard deviations reported at 1 sigma. The range of delta O-18 (from - 0.480 to 0.544 parts per thousand) falls within records of the Global Seawater Oxygen-18 Database at the Mariana Trench. The average O-17-excess value at the Mariana Trench is more negative than the average O-17-excess value of -5 +/- 4 (1 sigma) per meg in the only prior data set including deep ocean samples. The slope (lambda) of the three-isotope plot of Mariana Trench water is 0.521 +/- 0.003 (1 sigma), lower than lambda of 0.528 +/- 0.001 (1 sigma) for ocean water in the prior study. The new data set matches the prediction of the ocean isotope mass balance model, suggesting that it may represent a more appropriate ocean endmember for triple oxygen isotope thermometry. The O-17-excess of ocean water with respect to Vienna Standard Mean Ocean Water (VSMOW) is recognized to be a necessary correction in quantifying gross oxygen productivity of euphotic regions and relative humidity of moisture source regions.