Abstract: The electro-chemical behavior of phenophthalein has been studied in mercury film electrode,and an electro-chemical method for the determination of phenophthalein was established.In a buffer solution of NH₃NH₄Cl(pH 10),a reduction peak at -0.98 V(VS.Ag/AgCl) was observed.There was a good linear relationship between the peak height and the concentration of phenophthalein within the rang of 3.0×10^-7~5.0×10^-5 mol/L.The method was rapid,simple and accurate,and can be used for the determination of phenophthalein.