[en] The conventional method for determination of thorium is established by EDTA complexometric titration in which xylenol orange is used to indicate the titration end point. This paper presents a novel technique to identify the complexometric titration end point. The technique is that Fe(Ⅲ)/Fe(Ⅱ) redox couple replaces the use of xylenol orange to detect the end point by potentiometry. Various factors possibly influencing the measurement of thorium by EDTA complexometric titration were studied, such as repeatability of titrated volume, pH value, the concentration of EDTA, buffer solution and so on. The optimum conditions for the determination of thorium by EDTA complexometric titration were obtained. A new methodology was established for precise and accurate determination of thorium with Fe(Ⅲ)/Fe(Ⅱ) detecting the titration end point by potentiometry. Under optimized conditions, 5 mg thorium can be determined precisely with a relative standard deviation of 0.1%(n = 6). (authors)