The inverse microemulsion polymerization of water-soluble acrylamide monomers within near-critical and supercritical alkane continuous phases provides a potential route for production of polymers with novel physical properties and at high reaction rates. In order to define conditions for a model polymerization process, the phase behavior of a nonionic surfactant/acrylamide/water system in near-critical and supercritical mixtures of ethane and propane was examined. Acrylamide was found to act as a cosurfactant due to its short chain length and hydrophilic character and this is probably coordinated about the surfactant's ethylene oxide units at the interface, rather than residing in the micelle core. The effects of temperature and composition on the phase behavior can be qualitatively explained from results on C{sub i}E{sub j}/water/alkane systems at atmospheric pressure by using either generalized ternary liquid-phase diagrams or a model that considers micelle-micelle and micelle-fluid interactions. Increasing pressure up to 550 bar acts to shrink the various two-component phase envelopes which govern the phase behavior of the multicomponent system and thus increases stability. At constant dispersed-phase composition and temperature, the clearing pressure of the system depends on the ethane/propane ratio only via the fluid density, possibly due to the nearly ideal mixing behavior of these two alkanesmore » at high pressure. The ramifications of the phase behavior for the choice of polymerization conditions are discussed.« less