The cage occupancy of hydrogen in the single-crystals of simple hydrogen hydrates and hydrogen + argon mixed-gas hydrates was investigated by means of in situ Raman spectroscopy under the three-phase (hydrate + water + fluid) equilibrium condition. In the equilibrium pressure region higher than approximately 25 MPa, four hydrogen cluster and argon competitively occupied the large cages of structure-II hydrogen + argon mixed-gas hydrates. In addition, Raman spectroscopic analysis at liquid nitrogen temperature (77 K) supports that the clusters of two, three, or four hydrogen molecules occupy large cages. (C) 2010 Elsevier B.V. All rights reserved.