Abstract: Establishes a new method for the determination of ionization equilibrium constant of HPMCT using ultraviolet spectrophotometry,and illustrates the details about the principle,steps,and data processings.The solvent,pH value of the solution and the wave length at which the absorbancy is determined are the three major factors influencing the result precision.The pKa value of HPMCT having esterification rate 32.34% in the solvent of ethanol-water was 4.77.UV spectrophotometry giving the ionization constants of HPMCT is based on two facts: firstly the trimellitate group is chromogenic,secondly the HPMCT can readily dissolve in ethanol-water.