Incipient hydrate, aqueous liquid, vapor equilibrium conditions of hydrates formed from mixtures containing carbon dioxide and hydrogen or carbon dioxide, hydrogen and propane were determined. The gas phase concentrations at equilibrium were also measured. The presence of carbon dioxide lowers the hydrate fort-nation pressure substantially compared to that for pure hydrogen. Addition of propane as a third substance reduces the pressure even further. An analysis of the pressure-temperature conditions based on the Clapeyron equation was also carried out to infer the structures formed. In both cases it is believed that hydrogen participates in the hydrate phase. (c) 2006 Elsevier B.V. All rights reserved.