The nucleation of methane hydrate in water - methane and two-layer water - decane - methane systems was studied using thermal analysis under methane pressure. The nucleation processes in the systems under consideration were represented as empirical survival curves. The effect of aggregate state of guest-rich phase on the methane hydrate nucleation process was evaluated. The experiments were carried out at constant supercooling (19.7 degrees C). The two-step shape of experimental survival curves was observed in the both studied systems. Based on the data obtained and literature data, it was assumed that in the case of Teflon containers steady-state methane hydrate nucleation occurred at interfaces of water - methane and water - decane saturated with methane while non-isothermal nucleation with a higher rate may be caused by the presence of a small quantity of microimpurities in the initial samples or two-step mechanism of the methane hydrate nucleation process. The comparison of the observed periods of non-stationarity during which nucleation rate takes constant value and calculated nucleation rates showed that the presence of a liquid phase rich with the hydrate-forming component promotes a steadier nucleation of the methane hydrate in the quiescent reactor and facilitates the formation of clusters in the surface layer (water - decane).