Reaction intermediates in the reaction of 4,4＇-thiodiphenol (TDP) and Epikote 828 (E 828) for the formation of the diglycidyl ether of bisphenol A (DGEBA) epoxy copolymers (ETP) were determined by using a combination of two methods, viz. gel permeation chromatography( GPC) and matrix-assisted laser desorption ionization/time-of-flight (MALDI-TOF) spectrometer. This combination provides the measurement of true molecular weight and distribution, which in turn, enables the structural identification of intermediates and determination of their respective contents. The content of ideal component, the E 828-(TDP-E 828)(n), n = I in ETP copolymer, was found to be 2.3, 13.0, and 34.4% for ratios of 1/1, 1/2, and 1/3 of TDP and E 828, respectively. The ETP epoxy copolymers also exhibited thermal improvement over E 828 and other DGEBA resins of the E 1000 series, as indicated by their higher glass transition temperature (T-g) in differential scanning calorimeter (DSC) and better thermostability in thermogravimetric analysis (TGA) measurements.