An investigation of the ternary phase prism for low molecular weight poly(ethylene oxide), squalane, and poly(ethylene oxide-b-ethylenepropylene) is presented. Comparisons are made between this ternary polymer system and classic water/oil/surfactant mixtures to establish further a universal phase diagram description for amphiphilic systems. A combination of visual isothermal measurements, small-angle X-ray scattering, small-angle neutron scattering, and dynamical mechanical spectrometry was used to characterize phases and determine phase boundaries. A rich phase diagram was revealed, including most of the equilibrium liquid crystalline phases associated with diblock copolymers, regions of two-phase and three-phase coexistence, and a bicontinuous microemulsion. Differences between this polymer phase diagram and those from water/oil/surfactant systems highlight the strong effect of water in the latter.