Interchange reaction between poly(ethylene terephthalate) and poly(ethylene succinate) were effected by heat treatment above the melting temperatures. Based on $^1H-NMR$, DSC and X-ray analyses, the occurrence of interchange reaction to form P(ET/ES) copolymers were confirmed. Triad analysis of the $^1H-NMR$ spectra revealed that the average sequence lengths of copolymers decrease with heat treatment time down to 2~3. This suggests that, as interchange reaction proceeds, the physical mixture is changed to block copolymers and ultimately to random copolymers. Copolymers have a single $T_{g}$ and $T_{m}$ when heat treated for over 10 min at 28$0^{circ}C$, regardless of the composition. As heat treatment time increases, the glass transition temperature increases and the melting temperature decreases with a simultaneous decrease in the enthalpy of melting. When samples of similar ET and ES composition are heat treated for extended periods no melting peaks can be observed, suggesting the crystallizability of the random copolymers is low. The wide angle X-ray diffraction pattern of P(ET/ES) copolymers were different from what might be expected from a mixture of PET and PES.