Cationic dyeable modifiers (CDM) for polyethylene terephthalate (PET), bis(-2-hydroxyethyl)-5-sodium sulfoisophthalate (HESP) or 5-sodium sulfodimethyl isophthalate(MSP) was reacted with bis(-2-hydroxyethyl)-terephthalate (BHET). The reaction variables were temperature, time and the concentration of the CDM. The reaction coefficient (k) were determined from the slope of the plot of In (1-Y/100)vs. t, because it was investigated as 1st order reaction. The reaction coefficient was used for the calculation of the activation energy. The activation energy between BHET and HESP was lower than that between BHET and MSP because the intramolecular hydrogen bonding was predominant in both BHET and HESP. The hydrogen bonds might increase electron density of oxygen in -OH group and reduce the electron density of the ester carbonyl carbon, resulting easier access of the -OH group to carbonyl carbon at the rate determining step.