The electrocatalytic activity of cobalt tetra-2-mercaptopyrimidylphthalocyanine-modified glassy carbon electrode (CoTMPyr-Pc-GCE) toward gluthathione, L-cysteine and 2-mercaptoethanol has been investigated. CoTMPyrPc-GCE provides a significant improvement on the reported oxidation potential Of L-cysteine in pH 4.0 phosphate buffered solution. The oxidation peak potential (E-p), for L-cysteine, was 0.15V - a relatively lowered oxidation potential compared to reported phthalocyanine complexes. The electrode prepared by drop-dry/thermal annealing method was very stable and sensitive over a long period of time. The stability has been attributed to the thermal annealing and the structure of the mercaptopyrimidine on the periphery of the CoPc. (c) 2006 Elsevier B.V. All rights reserved.