A polarographic technic and Starkey''s alkali method have been applied to the quantitative analysis of thiosulfate, trithionate and pentathionate and to the quantitative analysis of tetrathionate in filtrates of bacterial cultures grown in an inorganic medium containing sodium thiosulfate. Five strains of autotrophic thiobacilli and one strain of a heterotrophic bacterium produced polythionates. During thiosulfate oxidation by two autotrophic strains which resembled Thiobacillus thioparus, tetrathionate, small amounts of trithionate, pentathionate and abundant elemental sulfur were produced and the pH fell from c. 7.0 to c. 3.0. In cultures of one of the three autotrophic strains which resembled T. thiocyanoxidans, about half as much tetrathionate, with even smaller amounts of trithionate and pentathionate, was found; abundant sulfur was precipitated and the pH fell to c. 4.0. In cultures of the two other strains resembling _T_. thiocyanoxidans, small amounts of tetrathionate were detected only occasionally; trithionate and pentathionate were not found, sulfur was deposited and the pH fell to c. 4.5. Much tetrathionate, as well as trithionate and pentathionate, were all readily identified in cultures of the heterotrophic organism; the pH rose to c. 8.7 and no sulfur was formed.