We investigate the geometric and electronic structure of p-phenylenevinylene oligomers substituted by electron-donor (methoxy) and/or -acceptor (cyano) groups; polymers such as poly(p-phenylenevinylene) and its derivatives have recently attracted much interest as active layers in light-emitting diodes. Geometry optimizations are carried out at the Hartree-Fock semiempirical Austin Model 1 (AM1) level; the electronic properties are derived from intermediate neglect of differential overlap (INDO) calculations, coupled to a single-configuration interaction scheme. We pay particular attention to the influence of the number and location of the donor and acceptor substituents, on the characteristics of the highest occupied and lowest unoccupied molecular levels. We simulate the UV-visible absorption spectra of the oligomers and describe the effect of substitution on the nature of the different absorption features.