In an effort to study sequence ordering during annealing in semicrystalline copolyesters, the carbonyl carbon of p-hydroxybenzoic acid was C-13 tagged and then solution polymerized with 2-hydroxy-6-naphthoic acid to form a low molecular weight, 50/50, random copolyester. The random copolyester was annealed at 40 deg below the crystal nematic transition temperature (T-cn). DSC results indicated a large increase in crystallinity; however C-13 NMR analysis showed no change in the benzoic-naphthoic (BN) and benzoic-benzoic (BB) dad sequence peak ratios. When the random copolyester was annealed near the T-cn, DSC results indicated a large increase in the transition temperature, no increase in crystallinity, and a shift in the BN:BB peak ratio indicating a significant increase in alternating sequences. On the basis of these results, the dramatic increase in transition temperature during annealing near To is best interpreted as sequence ordering via interchain transesterification reactions within the existing crystalline regions.