AbstractThis problem was investigated especially on account of the great number of controversies found in the literature. For a historical review reference is made to the papers of Michel2) and Jung3). A photoelectric colorimeter and a photoelectric spectrometer being of essential importance in this investigation, they are described first. Michel's observation that SHb contains per equivalent one sulphur atom more than normal hemoglobin, was confirmed by analysis of SHb prepared in vivo. This extra sulphur can be split off with bromine, but not with acid or alkali. The molecular weight of SHb is equal to that of normal hemoglobin (68000). By the action of dilute acid about five times as much iron is split off from SHb as from normal hemoglobin (“easily detached iron”). The absorption spectra of several derivatives of SHb are given. It is shown that potassium ferricyanide oxidizes the bivalent iron of SHb to trivalent iron. The compound formed can best be called ferrisulfhemoglobin. A rough calculation of the oxidation potential is given. The action of KCN on ferri‐SHb causes the extra sulphur to be split off from SHb. The spectrum of the compound formed is very similar to that of cyanhemoglobin. An analogous fact is found for the porphyrin and the hemochromogen which can be prepared from SHb. The difference in structure between globin hemochromogen and hemoglobin is discussed. The product formed by the action of hydrogen peroxide and ammonium sulphide on hemoglobin is not, as Michel presumes SHb, but it is probably identical with the verdohemochromogen found by Jung. Experiments are described which show that the empirical reaction equation for the formation of SHb is:\documentclass{article}\pagestyle{empty}\begin{document}\[{\rm Hb}+{\rm H}_2{\rm S}+{\rm O}_2={\rm SHb + 2H}_{\rm 2}{\rm O}\]\end{document}Further, an explanation is given of the fact that it is impossible to convert more than 75% of the Hb into SHb. In conclusion a hypothesis is given concerning the structure o