On investigating the generation of microcellular polyurethane foam via reaction in carbon dioxide, we have observed that common polyurethane precursors are CO2, miscible, whereas typically fluorinated compounds or specially designed surfactants are needed to solubilize polymers in CO2. Both isocyanates and polyols are CO2-miscible at workable pressures and temperatures and in useful concentrations to allow generation of polyurethane foams in CO2. By characterizing the phase behavior of several series of propylene oxide and ethylene oxide polyols, we have observed that the combined effects of molecular weight and hydroxyl number fix the location of the phase separation pressures. In general, lower molecular weights and lower hydroxyl mole fractions produce phase boundaries at relatively lower pressures in carbon dioxide. It also has been shown that CO2-soluble compounds may have a compatibilizing effect on less CO2-soluble materials.