Cryoscopic and conductimetric studies of a variety of compounds containing S—O bonds have shown that dimethyl sulphoxide behaves as a strong base in sulphuric acid whereas dialkyl sulphones behave as weak bases. In contrast diaryl sulphones and aryl sulphonic acids behave as non-electrolytes.A correlation is established between the basicities of the alkyl sulphones, H2SO4, and the HSO4− ion, and their sulphur–oxygen stretching frequencies. This relation is used to predict the basicities of other compounds containing S—O bonds. In particular it is shown that the basicity of monomeric sulphur trioxide is similar to that of sulphuric acid, and sulphamide is shown to behave as a strong base in sulphuric acid, which implies protonation on a nitrogen atom rather than on oxygen, since the S—O stretching frequencies indicate only weakly basic behavior for O-protonation.