Because gas hydrate is preferentially enriched in the heavy water isotopes, the δ18O and δD values of pore waters collected from gas hydrate--bearing sediment can provide information on the abundance and mechanisms of gas hydrate formation. Pore waters sampled from deep-seated (40 to 125 mbsf) gas hydrate deposits in Hydrate Ridge during ODP Leg 204 show depletion in dissolved Cl- and enrichments in 18O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (αO = 1.0025 and αH = 1.022) estimated from an extensive data set (n = 30 samples) correspond to experimentally determined values. In contrast, pore waters from shallow samples (<25 mbsf) at the ridge summit (n = 32) are highly enriched in dissolved Cl- and depleted in 18O and D, consistent with formation of massive gas hydrate deposits at rates faster than those at which these anomalies would be removed by advection or diffusion. The water isotopic fractionation factors in the brine are significantly lower than those experimentally determined, with αO of 1.0010 (average value of 1.0012) and αH of 1.008 (average value of 1.008). We discuss several factors that may be causing this anomalous fractionation and suggest that low gas occupancy in hydrate lattice (high hydration number) may be responsible for the observed small fractionation. If this were the case, the oxygen and hydrogen fractionation may serve as an indicator of hydration number during formation of gas hydrate in natural systems.