The thermal behaviour of a linear polyethylene (LPE) and of homogeneous ethylene-1-octene copolymers (2.1, 5.2 and 8.0 mol% of 1-octene) was investigated at high pressures by use of a high-pressure differential scanning colorimetry (DSC). The heating curves of LPE at pressures above 400 MPa reveal a three-peak pattern (peaks denoted by I, II, III at increasing temperature). It reflects the melting of orthorhombic folded chain crystals (I); the melting of extended chain crystals, superposed on the orthorhembic-hexagonal solid-solid transition (II) and the melting of the hexagonal phase (III). Introduction of a small amount of 1-octene results in a similar multiple-peak pattern but with a weakened high-temperature peak. With increasing comonomer content this three-peak pattern evolves over a two-peak pattern (I, II) into a single-peaked endotherm (I).