AbstractThe design of a differential thermal analysis apparatus for use at elevated pressure is described. Experiments on melting and crystallization of folded‐chain crystals of polyethylene and poly(ethylene–butene‐1) copolymer, and melting of extended‐chain polyethylene crystals have been conducted at pressures up to 4200 bars. The precision in transition temperature measurement was ±1°C. The Clausius‐Clapeyron equation predicts the melting point increase with pressure at atmospheric pressure to be 32.0°C/kb. The melting point depression due to copolymerization remained constant over the complete pressure range analyzed on the poly(ethylene–butene‐1) used in this study. Crystallization of polyethylene is retarded at elevated pressures, and a 50% larger degree of supercooling is necessary at 5000 bars to give a crystallization rate equal to that observed at atmospheric pressure. The difference in melting point between folded‐chain and extended‐chain polyethylene increases from 8.4°C at 1 bar t