Clathrate hydrate formation in the system methane + 3-methyl-1-butanol + water is demonstrated. The data of four-phase-equilibria for methane + 3-methyl-1-butanol + hydrate + water equilibria are determined in the temperature range of 273-282 K. The equilibrium pressures at temperatures below 279 K are lower, by 0.4 MPa at most, than those of structure I hydrate formed in the methane + water system without 3-methly-1-butanol, suggesting the formation of a hydrate different from structure I methane hydrate. Conversely, at temperatures above 279 K, the equilibrium pressures are higher by 0.2 MPa than those of the methane structure I hydrate, suggesting the thermodynamically inhibited formation of structure I methane hydrate. To confirm the crystallographic structures of the hydrates, X-ray diffraction and Raman-spectroscopy measurements were performed with hydrate crystal samples prepared at two conditions: at 274.0 K and 2.55 MPa, and at 279.5 K and 6.30 MPa. The measurements revealed a structure II hydrate formation at the lower temperature, contrary to the expectation of the structure H hydrate formation. The hydrate formed at the higher temperature was determined to be of structure I. (C) 2004 Elsevier B.V. All rights reserved.