Through isobaric temperature ramping experiments, the solubility of pure methane, ethane, propane, carbon dioxide gases and a methane-propane gas mixture in pure liquid water has been measured. The experiments are conducted at low temperatures and pressures corresponding to the clathrate hydrate formation and decomposition region. The inhibitory effect of a 10 weight percent methanol aqueous solution and a 0.5 weight percent polyvinylpyrrolidone aqueous solution on the hydrate formation and decomposition conditions has been estimated. A study of the pH-induced change in the hydrate stability has also been performed. The isobaric solubility data obtained show a significant divergence from Henry's law prior to and during hydrate formation. A molecular mechanism of hydrate nucleation is hypothesized, based on an analysis of the gas supersaturation observed and the current knowledge on the structure of liquid water.