Successful modeling of the chemistry of the troposphere and stratosphere requires accurate knowledge of the production rate of the OH molecule, since it occupies a central position in a number of chemical cycles. The reaction between O(1D) and H2O is believed to be the principal primary OH source, and an evaluation of the literature shows a discrepancy of about a factor of two between two of the most recent rate coefficients for this reaction. We have generated O(1D) from pulsed 1600 ¿ photolysis of O2, and observed the 6300 ¿ O(1D→3P) transition. The rate coefficient for H2O quenching of O(1D) has been directly determined, and found to be (2.6¿0.5) ¿10-10 cm3 molec-1 s-1 at 300 ¿K, in good agreement with the measurements of Streit, Howard, Schmeltekopf, Davidson, and Schiff.