The reaction of hydrazinium ion (N2H5+) with [Ru-III(HEDTA)(H2O)] (EDTA = ethylenediaminetetraacetate) is studied in aqueous solution in the pH range 1.0-4.5 at 25 degrees C by spectrophotometry and voltammetry. The products of the reaction, [Ru-III(HEDTA)(N2H5)](+) and [Ru-III(EDTA)(N2H5)], stable at pH < 4.5, are found to be formed with the second-order rate constants 5.63 X 10(-3) and 0.576 M(-1) s(-1), respectively. The former is also prepared and characterized by physicochemical methods. Reduction of the coordinated hydrazine to 2 equiv of ammonia is observed at E(1/2) similar to +0.035 V vs Ag/AgCl by voltammetry. These complexes are used as electrocatalysts for the reduction of hydrazine to ammonia. The turnover rates of formation of moles of ammonia per mole of catalyst per hour are 18.4 at pH 2.8 and 9.5 at pH 1.9 with 100% Coulombic efficiency, as calculated in constant-potential electrolyses at -0.05 V (Hg) vs SCE. A suitable mechanism for the catalytic reduction of hydrazine to ammonia in acidic media is proposed.