The formation of porous materials by thermally induced phase separation is discussed and two basic mechanisms are presented : interference of a liquid-liquid demixing with a glass transition and/or crystallization. The possibility of formation of porous ＇composite＇ fibres using a blend of two non-compatible polymers is illustrated. The formation of these fibres during extrusion leads to a fibrillar morphology with the porosity oriented along the fibre axis. The formation of porous syndiotactic polystyrene with different degrees of supramolecular organization is discussed. The transformation on heating of the initially amorphous, glassy material into the gamma-phase and then into the beta-phase makes it possible to maintain the demixed situation up to temperatures far above the Theta-temperature of this polymer-solvent system.