The electrocatalytic oxidation of hydrazine has been studied on glassy carbon electrodes modified by electrodeposition of iron(III)hexacyanoferrat(II) (Prussian Blue PB), by using cyclic voltammetry and the rotating disc technique. The oxidation of hydrazine to nitrogen occurs at the rising part of the oxidation wave of PB to Berlin Green at 0.7 V vs. Ag/AgCl where oxidation is not observed at the bare glassy carbon electrode. The dependence of the oxidation current on the hydrazine concentration and on the concentration of the redox centers FeII/III leads to the conclusion that the reaction is first order in both cases. From a Tafel plot a slope of 120 mV is obtained indicating a one-electron charge transfer process to be rate-limiting. The electrocatalytic efficiency increases with increasing pH up to pH 5 where PB becomes soluble; hence the reacting species is the N2H5+ ion. From the fact that the oxidation current increases with increasing film thickness it is concluded that the electrocatalytic reaction occurs inside the PB lattice. Further studies are reported concerning the determination of the diffusion coefficient of hydrazine in PB and the interpretation of the results within the framework of the Saveant-Andrieux model.