Different bacterial cell fractions of Thiobacillus neapolitanus were examined in order to localize the active sites for thiosulphate and sulphite oxidation. Difference spectra of the fractions were made to determine the level at which electrons from sulphite and thiosulphate enter the respiratory chain. Active sites for thiosulphate are probably strictly connected with cell membranes. Thiosulphate and sulphite reduced cytochome b and c in bacterial cell extracts. It has been found that thiosulphate oxidation is accompanied by production of tetrathionate and trithionate.