Polystyrene-block-polyisoprene forms micelles in supercritical and subcritical propane upon cooling and decompression. These micelles decompose upon heating and compression. At constant polymer concentration, the micellization points, both isobaric and isothermal, fall around a decreasing boundary curve in pressure-temperature coordinates. This micellization boundary curve lies above the copolymer cloud-point curve and below the free-polystyrene cloud-point curve. At the onset of micellization, a trace of free polystyrene can cause a characteristic scattering peak as it precipitates upon cooling or decompression or both.