4,4',7,7'-tetrachlorothioindigo (TCTI) and 4,4',5,5',7,7'-hexachlorothioindigo (HCTI) are thionated derivatives of indigo and are known as reddish purple pigments and photoconductors. Although the molecular structures of these compounds are quite similar, TCTI is less soluble in organic solvents and more photoconductive than HCTI by about one order of magnitude. Obviously, intermolecular interactions are responsible for these phenomena. Therefore, the intermolecular forces in TCTI and HCTI have been investigated in terms of an energy partition analysis based on semi-empirical molecular orbital calculations. The results indicate that there are intermolecular CH…O hydrogen bonds in both TCTI and HCTI. Especially, the H-bond network of TCTI is found to be quite significant and is correlated with its high insolubility in solvents.