We calculate the solubility of methane gas over a range of pressure and temperature. The gas is dissolved in liquid water, which coexists with free gas at high temperature or solid hydrate at low temperature and high pressure. We show that solubility is significantly altered by the presence or absence of the hydrate phase. When hydrate is absent at high temperatures, our calculations reproduce experimentally observed increases in solubility with decreasing temperature. When hydrate is present, however, we find that the gas solubility decreases sharply with decreasing temperature. Such an abrupt decrease in solubility permits hydrate to crystallize directly from the aqueous solution, without the need of any free gas. This result has important implications for the formation of gas hydrate in marine environments, where the gas supply may not be sufficient to provide free gas. We apply our calculations at typical pressure and temperature conditions in marine sediments to establish the gas concentration needed to stabilize hydrate. Estimates of the vertical distribution of hydrate in marine sediments and the rate of accumulation are obtained using simple models of hydrate formation.¿ 1997 American Geophysical Union