A chemically modified electrode for the amperometric detection of hydrogen peroxide is described. By electrodeposition of a copper heptacyanonitrosylferrate(II) (CuHNF) film on the surface of a glassy carbon electrode, the overvoltages for both oxidation and reduction of hydrogen peroxide can be drastically decreased. Cyclic voltammetry at the resulting modified electrode reveals the peresence of two pairs of redox peaks corresponding to the Cu(I)-Cu(II) and Fe(II)(CN) 5NO-Fe(III)(CN) 5NO redox couples, respectively. The modified electrode is less prone to fouling and shows enhanced sensitivity for hydrogen peroxide. Various electrochemical response characteristics were investigated. When applied as an electrochemical sensor in a flow-injection system, the modified electrode permitted detection of hydrogen peroxide at lower potentials than at the unmodified electrode surface. This greatly improved the selectivity. A linear amperometric response range with hydrogen peroxide was obtained between 0.4 μM and 1 mM. The relative standard deviation for 30 repetitive injections with a 10 μM hydrogen peroxide solution over a 30-min period was less than 4%.