Abstract: In a dil.H2SO4 medium,sensitive adsorptive polarographic wave given by the nitrisation product of the reaction between methyl orange and nitrite ion was observed in the potential range of -0.20 V to -0.40 V (vs.SCE).It was found than linear relationship was obtained between the 2nd derivative of the peak height (i″p) measured at -262 mV (vs.SCE) and the concentration of nitrite ion in the range of 4.0×10-5-5.0×10-2mg·L-1,with a detection limit of 1.0×10-5mg·L-1,which is much lower than that of conventional polarography.Precise and accurate results were obtained in the application of this method to the analysis of river water and tap water samples (RSD<3.5%,recovery:98.5%-100.4%).A brief study on the mechanism of the nitrisation reaction and the electrode reaction was also given.