Isomorphism of copolymers of ethylene terephthalate (ET) and 1,4-cyclohexenedimethylene terephthalate (CT) was analysed by high-resolution solid-state C-13 nuclear magnetic resonance (n.m.r.) spectroscopy. The ET/CT copolymer system was confirmed to have random comonomer sequence distribution. The trans/cis isomer compositions of the cyclohexene group in the CT unit were almost constant for all samples, giving a ratio of 67/33. The CT contents in the crystalline phases were determined by high-resolution solid-state C-13 n.m.r. spectroscopy. The crystalline phases consisted of only ET components in the composition range of 80-100 mol% ET, whereas those in the composition range of 66-100% CT adopted the incorporation of the comonomer units in the PCT crystal.