We describe a scheme to determine the major sulfur species found in marine porewaters. With polarographic techniques, it is possible to measure thiosulfate, sulfite, bisulfide, and polysulfide ions at a mercury electrode. Polysulfide ions, Sx2−, can be considered to be composed of one sulfur in the 2− oxidation state, S(2−), and the remaining (x − 1) sulfurs in the zero‐valent oxidation state, S(0). The number of sulfur atoms in each is measurable. Tetrathionate and other polythionates can be measured as well but have not been detected in porewaters studied to date. Salt marsh and subtidal porewater profiles contain significant concentrations of thiosulfate, bisulfide, and polysulfide. The ratio of S(2−) to S(0) is higher in subtidal porewaters than in salt marsh porewaters and indicates the importance of zero‐valent sulfur to the biogeochemical processes prevalent in the salt marsh ecosystem. The S(0) concentrations in porewaters from salt marsh sediments are greater than those predicted from equilibrium calculations.