The system carbon dioxide-acetylene at cryogenic temperatures has been reinvestigated. Measurements have been made of the vapor pressure as functions of composition and temperature, and of infrared spectra of cryofilms at 90 K as a function of composition. It was concluded that, under suitable conditions, carbon dioxide and acetylene can co-crystallize to form cubic crystals of the stoichiometry CO2. C2H2, and that this phase is metastable with respect to the two pure phases. This metastability was confirmed by following the decomposition of CO2. C2H2 at 90 K as a function of time, and the fractional decomposition was found to vary sigmoidally with time. The kinetics of the decomposition are roughly described by a model in which the rate increases as product nuclei grow and decreases as the reactant is depleted (Avrami model).