A cobalt phthalocyanine-modified boron-doped diamond (CoPc-BDD) electrode was fabricated for a sensitive detection of hydrogen peroxide. The hydrogen-terminated BDD electrode surface was terminated with pyridine moieties via photochemical modification with 4-vinylpyridine followed by immobilization of CoPc on the surface by immersion in a CoPc solution. X-ray photoelectron spectroscopy studies suggested that the coordination of the surface pyridine and the Co atom in CoPc contributes to surface modification. Electrochemical detection of hydrogen peroxide at the modified electrode was also investigated. Cyclic voltammetry confirmed that the CoPc-BDD electrode exhibited a catalytic activity for the electrochemical oxidation of hydrogen peroxide. Using a flow-injection analysis-electrochemical detection system, we demonstrated that the CoPc-BDD electrode is well suited for the detection of hydrogen peroxide, with a limit of detection of 4.9 nM. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3211799] All rights reserved.