The influence of cross-conjugation on the optical and electrochemical properties Of donor acceptor copolymers is investigated. Isoindigo, substituted at the 6,6' or the 5,5' positions, and thieno[3,2-b]thiopliene and thieno[2,3-]thiophene ate taken as conjugated and cross-conjugated electron-deficient and electron-rich building blocks from which four isomeric donor acceptor polymers were synthesized. Introducing cross-conjugation in isoindigo, has only a small effect on the electrochemical band gap and on the onset of the absorption, which remains in the near-infrared. The optical absorption spectra, however, differ strongly because cross-conjugation strongly reduces the absorption coefficient. DFT calculations confirm that the transition to the lowest excited singlet state has a small oscillator strength in cross-conjugated isoindigo model Compounds. Cross-conjugation in thienothiophene exerts a different effect. It causes a moderate but distinct blue-shift of the optical absorption and a deeper HOMO energy level.