Abstract: The frequency of the Raman band for methane dissolved in water indicates clustering of water molecules around methane molecules. By monitoring trends in the position of this band with pressure, it was determined that the clusters resemble CH4 trapped in the 20-coordinated small 512 cavity, the basic building block of clathrate hydrates. These methane-water clusters act as precursors to hydrate cavity formation and their transformation to methane trapped in cavities of sI hydrate was monitored using time resolved Raman spectroscopy. The ratio of number of large to small cavities was obtained as a function of time. The formation of the large 51262 cavity was determined to be slower than for the small 512 cavity. Based on these results, a mechanistic model of methane hydrate formation from the aqueous phase is proposed.