Previous authors have suggested that volume-pressure plots can be used to discriminate between pressurized cores with CH4 hydrate and those with only dissolved CH4. Volume-pressure plots were constructed for 20 PCS cores at Sites 994, 995, 996, and 997. Most of these plots conform to expected volume and pressure changes for cores with low CH4 concentrations at 0ºC and no gas hydrate. This observation contrasts with other information that indicates relatively high CH4 concentrations and gas hydrate in sediment at these sites at in situ conditions. We conclude that conditions at the start of gas-release experiments with the PCS are sufficiently different than those at in situ conditions. In particular, CH4 concentrations decrease by at least 70% because of dilution with borehole water within the PCS. This dilution is of sufficient magnitude that CH4 hydrate-bearing sediment cores will show no indications of hydrate during PCS degassing experiments, because the hydrate will have decomposed during equilibration before degassing. The PCS cannot be used to detect unambiguously the presence (or absence) of gas hydrate in sediment at in situ conditions through volume-pressure (or time-pressure) relationships alone. However, the total gas volume released from a pressurized core of known porosity will render pore-volume gas quantities that, in conjunction with appropriate phase diagrams, can be used to estimate the abundance of gas hydrate and free gas (Dickens et al., 1997).