Electrodeposited Fe(III) meso-Tetra(4-carboxyphenyl) porphyrin with cyanide ligands coordinated to the central iron ion is being investigated as a possible material capable of selectively oxidizing H 2S in the presence of CO and NO in physiologically relevant solutions. Preliminary results suggest this to be a very attractive material as it oxidizes H 2S at low potentials while exhibiting minimal activity towards both CO and NO. However certain results suggest that the Fe may not be completely coordinated with cyanide ligands. This may explain the limitations of the material at elevated H 2S concentrations and imperfect selectivity over the interfering gases. Additionally, a cathodic-post run treatment is suggested in order to maintain the electrochemical activity of the porphyrin-modified electrode surface by stripping any S 0 that deposits on the electrode surface during the H 2S oxidation.