An understanding of the in situ thermal properties (e.g., thermal conductivity and thermal diffusivity) of gas hydrates in marine sediments is important for analyzing the effects of gas hydrate on thermal gradients, for accurately estimating temperatures at the depth of the bottom simulating reflector, and for calibrating empirical relationships with other physical properties, such as seismic wave speed. Previous laboratory thermal conductivity measurements on gas hydrates have primarily focused on propane and tetrahydrofuran hydrates and have sometimes been carried out at pressures and temperatures not representative of in situ marine conditions. In this study, we report on preliminary measurements of the thermal conductivity of pure methane gas hydrate and of gas hydrate and sand mixtures at simulated in situ conditions. We use the USGS-Menlo Park gas hydrate synthesis apparatus to form gas hydrate in a pressure chamber by mixing and subsequently heating sieved, granular ice (180-250 ?m) and cold, pressurized methane gas. For the experiments that include sediment, the gas hydrate is formed from a homogeneous mixture of equal volumes of 100 ?m quartz sand and granular ice. The synthesis technique yields well-characterized samples with porosities of 29% (pure methane hydrate) and 38.5% (sand and methane hydrate). Using the needle probe method of von Herzen and Maxwell, we have measured thermal conductivities of the samples at a range of pressures (13-30 MPa) and temperatures (-5 to 5?C) within the methane hydrate stability field. Preliminary results yield an estimated thermal conductivity of 0.81-0.87 W/mK for mixtures of quartz sand and methane hydrate and methane gas. In addition to providing control over hydrate and sediment mixture ratios, the measurement apparatus is also well suited for pore fluid injection. Using an effective medium approach applied to laboratory measurements, we can assess the effects of varying methane hydrate concentrations and pore fluid compositions on the thermal conductivities of a range of hydrate/sediment/pore fluid mixtures.