The dissociation of methane hydrate at high pressure is studied by <i>in situ</i> Raman spectroscopy in a diamond-anvil cell. As for the Raman spectrum of sI methane hydrate, the v<sub>1</sub> band of CH<sub>4</sub> is split into two peaks v′<sub>1</sub> and v″<sub>1</sub>, indicating the partitioning of CH<sub>4</sub> between the large (5<sup>12</sup>6<sup>2</sup>) and small (5<sup>12</sup>) cavities, respectively. With increasing temperature, the intensity ratio of <i>I</i>v′<sub>1</sub>/<i>I</i>v″<sub>1</sub> decreases obviously, and the d(<i>I</i>v′<sub>1</sub>/<i>I</i>v″<sub>1</sub>)/d<i>T</i> is −0.079 K<sup>−1</sup>. Additionally, the v<sub>1</sub> band of the dissolved CH<sub>4</sub> is close to v″<sub>1</sub> of the CH<sub>4</sub> molecule engaged in the small 5<sup>12</sup> cavity. This implies that, in the initial stage of hydrate formation, the abundance of small 5<sup>12</sup> cavities is greater than that of large 5<sup>12</sup>6<sup>2</sup> cavities.

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