The monoclinic modification of linear polyethylene has been prepared by battering samples of the orthorhombic modification. The X-ray diffraction pattern, the CP/MAS C-13 n.m.r. spectrum, and the thermal stability of the phase are re-examined. The efficacy of WAXS and n.m.r. analyses in identifying this phase in the presence of the orthorhombic and amorphous phases and in reactor powders and PE copolymers is discussed.