Cyclic voltammetry was used to investigate the electrochemical behavior of hydrogen sulfide, methanethiol, ethanethiol and dimethyldisulfide at a cobalt phthalocyanine modified screen-printed carbon electrode (SPE) in phosphate electrolytes of different pH values. The thiols and hydrogen sulfide were found to undergo electrocatalytic oxidation while dimethyldisulfide exhibited an electrocatalytic reduction reaction. These processes were considered to involve the central cobalt(ii) ion in the phthalocyanine macrocycle. The possibility of monitoring the thiols by exploiting the electrocatalytic oxidation process and DMDS by the electrocatalytic reduction process was examined by amperometry. In one approach a bulk phosphate electrolyte, pH 12.0, was employed. The amperometric behavior of the low boiling liquids ethanethiol and DMDS were studied by making standard additions to a stirred solution; both species showed rapid response times at applied potentials of +O. 15 V and -0.40 V, respectively. The gaseous thiols, hydrogen sulfide and methanethiol, were examined by bubbling these through the same bulk electrolyte and again rapid, reproducible and reversible signals were obtained. In another approach a gel was prepared by adding agarose to the same phosphate electrolyte and a thin layer was deposited over the working and counter: referencc electrodes of a screen-printed strip. This sensor showed favorable electrochemical characteristics for the gaseous species examined which suggested that the device had promise as an amperometric gas sensor.