As the result of some preliminary observations by Stearns1, an extensive study was made by Wyman and Brode2 of the absorption spectra of solutions of several thioindigo dyes in benzene and chloroform under various conditions of illumination. Evidence was obtained that in solution thioindigo and its derivatives exist in equilibrium as two forms (cis and trans), which give rise to separate absorption bands in the visible region. X-ray diffraction studies on crystalline thioindigo by Madame von Eller3 suggest, however, that in the solid state the dye is present in the trans form. It is thought that the electrostatic attraction between slight positive and negative charges on neighbouring sulphur and oxygen atoms, respectively, maintains the thioindigo molecule in the trans configuration and prevents isomerization4. Presumably in some cases these attractive forces will be weakened by solvents or by substitution in the thioindigo molecule.