The preparation of porous fibers from the quasi-ternary system poly(methyl methacrylate)/poly(ethyl methacrylate)-1-butanol is based on the combination of different liquid-liquid demixing processes that interfere with the vitrification process. Polymer-polymer demixing takes place at high temperature and interferes at lower temperature with polymer-solvent demixing and the vitrification of some of the formed phases. When these processes take place during extrusion, porous "composite" fibers are obtained. They consist of fibrils of poly(methyl methacrylate) coated with poly(ethyl methacrylate), and these fibrils are separated by micrometer-sized channels that extend along the fiber axis. The formation of these fibers is discussed within the framework of the phase behavior of the quasi-ternary system.