AbstractEthylene—propylene and ethylene—butene‐1 copolymers with up to 1.7 side groups per 100 carbons have been crystallized at 227°C. and under 4100–4900 atm. pressure. The resulting crystalline polymers are at least partially of extended‐chain crystal morphology. Comparison with the same polymers crystallized at atmospheric pressure, which gives folded‐chain crystal morphology, revealed: (1) a density higher by 0.008–0.019 g./cm.3depending on copolymer content; (2) a similar decrease of crystallinity with side group concentration; (3) a similar decrease of the beginning of melting from 125°C. for homopolymer to 65°C. for 1.7 side groups per 100 carbons; (4) a higher (138 ± 0.8°C.) experimental maximum melting point which, in contrast, is independent of copolymer content and seems to vary only with the fraction of low molecular weight material; (5) a decreasing amount of high‐melting crystals with increasing copolymer content (72–8%) and an increasing amount of low‐melting crystals (27–53%) with increasing copolymer content. In addition, superheating, which reached 5.5°C. for 50°C./min. heating rates, was detected. It was concluded that high‐pressure crystallization leads, at least for part of the crystals, to solid solution formation, while atmospheric pressure crystallization does not. Which mode of crystallization is achieved seems kinetically determined. Experimental techniques were dil