Three hydroxy-terminated poly(bisphenol A iso/terephthalate) copolymer blocks have been prepared with molecular weights of 2400 ± 5%, and isophthalate to terephthalate ratios of 25:75; 50:50; 75:25. Their properties have been determined. These blocks were then coupled with phosgene to give alternating polyester copolycarbonates of high molecular weight. A random copolycarbonate of 50:50 iso- and terephthalate blocks was also made. Physical properties were measured on the copolycarbonates. The glass transition temperature fell as the proportion of the isophthalate entity increased and at 50% isophthalate reached a value close to that of the pure isophthalate copolycarbonate. Tensile stress-strain curves indicated a changeover from near uniform extension (25% isophthalate) to the more normal type of necking behaviour with the isophthalate carbonates.