Abstract: Chemically modified electrodes were prepared by anodizing vitreous-carbon electrodes in a RuCl3 - K4Ru(CN)6 solution Such electrodes catalyse the oxidation of hydrazines in acidic and neutral media, and show improved stability and a broader pH range of application then previous phthalocyanine surfaces. The response of hydrazine, methylhydrazine and dimethylhydrazine were evaluated with respect to film preparation conditions, pH, potential scan rate, concentration. and other variables. The electrodes were used in a flow injection system. The calibration graph was rectilinear for 5 to 50 µM-hydrazine, and the detection limit was 0.2 µM. The coefficient of variation was 1.3% (n = 30).