Thermally stimulated current/relaxation map analysis (TSC/RMA) study on the T-g transition of the phosphate-containing epoxy copolymers reveals that the cooperative motion of the penta-coordinated phosphate group with 4,4'-thiodiphenol as the tribranched segment is the prime factor responsible for the lowering of the T-g for the copolymer. Thermal windowing technique of RMA leads to compensation search that produced with compensation points in "Z" structure. From this Z data, a convenient method is used for the verification of the T-g value. Thermokinetic data of enthalpy (Delta H) and entropy (Delta S) also provide additional supporting data for the determination of T-g based on the compensation search from the RMA measurement.