A new family of terephthalate-based copolyesters has been found to exhibit high impact resistance combined with good thermal properties, ultraviolet stability, optical clarity, and low color. These engineering thermoplastic compositions were prepared using conformationally rigid cis/trans-2,2,4,4-tetramethyl-1,3-cyclobutanediol [CBDO] and flexible C-2-C-4 aliphatic glycols. The copolymers were amorphous when the CBDO (similar to 50/50 cis/trans) content was about 40 to 90 mol % of total diol. Glass transition temperatures were 80-168 degrees C, depending on the proportion of rigid CBDO units. Impact resistance was inversely proportional to CBDO content, and notched Izod values as high as 1000 J/m were obtained. Both high T-g (>100 degrees C) and high impact (250-750 J/m) can be realized simultaneously for compositions containing about 50-80 mol % CBDO. Accelerated weathering indicated good inherent resistance of 1,3-propanediol/CBDO copolyterephthalate to yellowing under ultraviolet radiation. Dibutyltin oxide was more effective for transesterification of CBDO with dimethyl terephthalate than other typical catalysts. Better color and higher molecular weights were obtained with this catalyst when the flexible diol was 1,3-propanediol or 1,4-butanediol rather than ethylene glycol.