Carbon monoxide occurs in abundance throughout the cosmos, potentially in clathrate form, whereas on Earth, it forms a notable constituent of industrial flue gases. It has been proposed that hydrate technology could be used in CO2 separation from flue gases, and in subsea flue gas CO2 disposal. This-and the likely widespread occurrence of CO clathrates in the cosmos-means it is important that the phase behavior of CO hydrates is known. Here, we present experimental H-L-V (hydrate-liquid-vapor) equilibrium data for CO, CO-CO2, and CO-C3H8 (propane) clathrate hydrates. Data were generated by a reliable step-heating technique validated using measured data for CO2 and CH4 hydrates. Data for CO and CO-C3H8 clathrates have been used in the optimization of Kihara potential parameters for CO, reported here, facilitating the extension of a thermodynamic model to the prediction of CO hydrate equilibria. Model predictions are validated against independent experimental data for CO-CO2 (structure I) systems, with good agreement being observed. (c) 2005 American Institute of Chemical Engineers.