The crystallization behaviour of the poly(ethylene terephthalate-co-1,4-cyclohexylene dimethylene terephthalate) (P(ET/CT)) random copolyesters is studied by d.s.c. analysis. ET units can crystallize with complete rejection of the CT units from the crystals, whereas CT units can co-crystallize with incorporation of ET units to some extent, resulting in a minimum melting temperature at an intermediate composition of about 30-40 mol% CT. A good linear relation is found between the compositions at which the melting point exhibits a minimum experimentally in various random copolyesters and those at which the cohesive energies for two components estimated by group contribution methods are identical.