We study the low-lying two-photon excited states in nonlinear optical processes in poly(p-phenylene vinylene) (PPV) and its derivatives. We find that, within the Pariser-Parr-Pople (PPP) model, the mA(g) state in (unsubstituted) PPV is exactly the lowest two-photon excited state (i.e., the 2(1)A(g)(+) state). Due to the breaking of charge-conjugation symmetry, the 2A(g) in substituted PPVs, originating from the 1(1)A(g)(-,) is weakly two-photon allowed, while the mA(g) has its origin from the 2(1)A(g)(+), hence giving a strong peak in the two-photon absorption spectra. (C) 2003 Elsevier Science B.V. All rights reserved.