Hyperpolarizability beta of fused-ring molecules tetra-substituted by two ortho-positioned donor (D)-acceptor (A) were studied for three types Of C-2v isomers, using simplified sum-over-states (SOS) calculation, missing-orbital analysis (MOA), and homologous substitution model. The model is comprised of two molecules, one di-substituted by two D's, the other by two A's at the sites homologous as a whole to tetra-substitution. Ortho1-type with two D-A pairs at molecular x-edges showed larger beta(zxx) than that of the model, exhibiting charge-transfer (CT), nevertheless all isomer types could show CT enhancement according to resonant structures. The substitution model successfully described qualitatively magnitudes and signs for beta for all isomers. (C) 2002 Published by Elsevier Science B.V.