We report theoretical studies on the optical absorption in 2-methoxy-5-(2'-ethyl-hexyloxy)-distyryl benzene (MEH-DSB), We first produce the major absorption features existing in experimental results, in particular a weak band at 5.1 eV, which has not been observed in poly(p-phenylene vinylene) (PPV). We find that this band can appear only when the strength of intersite Coulomb interactions becomes smaller, in comparison with that in five-ring PPV oligomer with no substituents. Detailed analysis of wavefunctions related to the corresponding absorption bands will give correct assignments. (C) 2002 Elsevier Science B.V. All rights reserved.