Knowledge of the thermal properties of gas hydrates and sediments containing gas hydrates is essential for assessing their commercial potential for natural gas recovery and their possible factors in sea-floor stability and climate change. Unlike phase equilibrium properties of hydrates, little information is available on their thermal properties. A major experimental challenge in thermal property measurement is determining the composition of the sample being measured. This chapter describes work being performed at the National Energy Technology Laboratory to develop a means to reliably measure the thermal properties of hydrate and hydrate-containing samples, while facilitating characterization of the sample with minimal decomposition or disturbance. A transient plane source (TPS) technique for simultaneously determining thermal conductivity and thermal diffusivity has been adapted for use at high pressure for this purpose. The TPS element is mounted inside a specially designed cup assembly that not only holds and contains the sample, but can also serve as a sample compaction device. The cup assembly is contained inside a high-pressure vessel that not only facilitates measurements at in-situ conditions, but can also be used to form hydrate or hydrate-containing samples in contact with the TPS element. The part of the cup containing the TPS element can simply be pulled away from the hydrate sample to permit subsequent characterization of the part of the sample that was measured. The formation of uncompacted methane hydrate in the cup and measurement of its thermal properties are described. The recovery of the sample and characterization by Raman spectroscopy are also presented.