Homo- and copolymers of hydroxythiophene and cyanothiophene have been investigated by employing density functional theory with the aim of determining the effect of donor-acceptor substitution on the electronic structure. The band gap of the copolymer is 0.11 eV smaller than that of polythiophene. Bandwidths of valence and conduction bands are reduced by 0.22 and 0.36 eV compared to polybithiophene. Conductivity after p-and n-doping could therefore be less than that of polythiophene. All properties of the copolymer are averages between those of the homopolymers. The charge separation between hydroxy- and cyano-substituted rings is 0.12 a in the neutral state and 0.13 a and the dication. The ionization potential and electron affinity of poly(hydroxythiophene) are 1.78 and 1.63 eV smaller than those of poly(cyanothiophene). According to the donor-acceptor concept, a decrease in band gap and an increase in bandwidths compared to the homopolymers should have resulted. We rationalize the absence of band broadening with reduced interaction between fragments with very different energies in agreement with perturbation theory.