Abstract: A biological activity probe for the detection on-line of the presence of sulphide including an enzyme electrode; a counter electrode; a reference electrode; and if desired a corrosion electrode. The enzyme electrode includes an immobilised source of sulfide oxidase enzyme, together with a cofactor such as 1,1′-dimethylferrocene, immobilised in a water permeable non-conducting binder on a biologically inert electrically conducting substrate. The immobilised source of sulfide oxidase enzyme is either a chemoautotropic microorganism such as a Thiobacillus thioparus, or enzyme material recovered from a culture of a Thiobacillus thioparus.