Abstract: Ion chromatography was applied to the determination of trace Br^- in catholyte solution of electrolysis synthesis of tetramethyl phosphorus hydroxide from tetramethyl phosphorus bromide. The chromatography work conditons were as follow:IonPac AS11-HC separation column, clolumn temperature of 30℃, 30 mmol·L^-1 KOH solution as eluent,flow rate of 1.0 mL·min^-1, suppressor current of 75 mA. The linearity range of Br^- was 1.00-10.00 mg·L^-1, with detection limit (3s/k) of 0.028 5 mg·L^-1. Values of recovery obtained by standard addition method were in the range of 96.2%-111%, RSD (n=7) was less than 1.0%. The effects of different commercial membranes, electrolytic time, current density on Br^- content in catholyte solution were investigated. The results showed that, when JCM-Ⅱ homogeneous cation exchange membrane was used to separate the anode and cathode of the electrolytic cell, the current density was 60 mA·cm^-2 and the power was 60% of the theoretical power, Br^- content in the cathode solution was less.