Chemically modified electrodes, constructed by incorporating oxomolybdenum(V) phthalocyanine (OMo(V)(OH)Pc, Pc = phthalocyanine dianion) into graphite powder were used to catalyse the oxidation of cysteine. Solution catalysis of cysteine by oxomolybdenum(V) tetrasulfophthalocyanine, [OMo(V)(OH)TSPc](4-), was also investigated. A considerable reduction in overpotential for cysteine oxidation was observed. Cysteine oxidation occurred at 0.26 and 0.28 V vs. AgAgCl for catalysis by OMo(V)(OH)Pc and [OMo(V)(OH)TSPc](4-) respectively as opposed to 0.77 V vs. AgAgCl observed on CoPc chemically modified electrodes. The anodic peak currents vary linearly with cysteine concentration in the range 0.02 to 0.08 mol dm(-3) and 0.008 to 0.02 mol dm(-3) for [OMo(V)(OH)TSPc](4-) and OMo(V)Pc respectively.