Miscibility-induced sequential reordering (MISR) was investigated using a Monte Carlo simulation method in two systems, i.e., a ternary copolyester system and a binary copolyester system in the presence of a third noncondensation polymer. A homogeneous AB random copolymer/C homopolymer blend was used as an initial blend. Phase separation and ester-interchange reactions were performed simultaneously at a trial ratio of 100/1 using a one-site bond fluctuation model, MISR was monitored by calculating the number-average sequence length of each segment. In the case of the ternary copolyester system, ester-interchange reactions between segments A, B, and C were attempted under various attractive and repulsive interaction energies between the three. It was found that MISR of segment A was induced only when segment C was miscible with segment B. The more repulsive segments A and C are and the more attractive segments B and C are, the longer the sequence length of the segment A becomes. In the case of a binary copolyester system in the presence of a third noncondensation polymer, segment A or B is sequentially reordered, accompanied by phase separation of the blend, when segment C is miscible with either segment A or B, respectively. The more repulsive the interaction between segments A and C is and the more attractive the interaction between segments B and C is, the longer the sequence length of segments A and B becomes.