Substituent effects on the electronic structure of 23 biologically active 4-aminoaryl (4-substituted aryl) sulphones were investigated by means of ¹H n.m.r., ¹³C n.m.r., and i.r. spectroscopy, as well as by semiempirical all-valence CNDO/2 calculations, with and without sulphur d orbital participation. Good linear intercorrelations were found among the spectral data and between these and the computed electronic charges and the Hammett σ_p values. On this basis the substituent effects are interpreted in terms of electronic charge perturbation, which is linearly transmitted from the substituent to the whole molecule, bridging SO₂ group included. The agreement between experimental and theoretical data is good and the trends do not depend on the inclusion or exclusion, in the calculations, of the sulphur d orbitals. Strong and linearly related σ–π electron interactions operate between C-1 and C-1′ in the C(1)SO₂C(1′) moiety; thus Koch and Moffitt's qualitative π-electron conjugation model for Case 2 type sulphones seems inadequate.