Ethanol (EtOH) is commonly used as a gas hydrate inhibitor in hydrocarbon production operations. However, a number of stable and metastable hydrate phases have been reported for the binary EtOH-H2O system at temperatures of <223 K, including a structure II type clathrate hydrate stable below 198 K. Here, we present experimental DTA and PVT phase equilibrium data for the binary ethanol-water and ternary ethanol-methane-water systems, respectively. Binary DTA data confirm the appearance of metastable EtOH hydrates above the established structure II clathrate peritectic transition. In the ternary system with methane, at XEtOH > 0.056, aqueous ethanol forms binary EtOH-CH 4clathrate hydrates stable over a wide PT range. In the HEtOH-CH 4+L+G region, this results in significantly less hydrate inhibition than would be expected from ice melting point depression. In the ice region, ethanol enclathration actually increases hydrate stability relative to the methane-water system; the HEtOH-CH4+L+G region being bounded by a univariant HEtOH-CH4+L+I+G quadruple point locus line at temperatures much higher than the typical HCH4+I+G boundary (or HCH 4+I+L+G quadruple point univariant locus in the presence of an aqueous hydrate inhibitor). Compositional analyses of the clathrate phase yields the formula 2.30CH4·0.66EtOH· 17H2O at 246.7 K and 3.68 MPa, which is consistent with structure II. Independent powder X-ray diffraction and Raman spectroscopic studies presented in an accompanying article in this journal issue confirm ethanol-methane clathrates to be of structure II type. ©2009 American Chemical Society.