AbstractDimer acid (D) modified copolyesters based on 1,4‐cyclohexane dimethanol (CHDM) and either terephthalic acid (T) or trans‐1,4‐cyclohexane dicarboxylic acid (C) were examined. Both series produced clear, flexible materials with physical properties changing regularly with changing modifier levels. Yield stress, elongation at break, and Young's modulus showed linear correlations of log property versus volume fraction of CHDM‐D segments in accord with the logarithmic rule of mixtures. Annealing the CHDM‐T‐based copolyesters caused little or no change in properties. Annealing the CHDM‐C‐based copolyesters gave similar results except for a large decrease in the modulus—the higher the dimer acid level, the larger the decrease in modulus. The density measurements, the x‐ray pattern, and the dynamic mechanical analysis at 110 Hz before and after the annealing treatment supported the hypothesis that the modulus decrease resulted from domain perfection in the copolyester induced by the annealing process. Transmission electron‐microscope examination of osmium tetroxide‐stained films supported this hypothesis, although initial results were not repeatable.