Sulphide, sulphite and thiosulphate can be determined separately or in admixture, with thallic perchlorate or sulphate in acid medium. A sample solution is rendered approximately 0.5 M in acid, 5 ml of 0.05 M KI are added and the solution is titrated to a starch end-point with thallium(III) solution. In another method an acid sample solution is titrated with thallium(III) or iodine solution in the presence of indigo carmine indicator. The end-point is improved in the presence of Co(II).