Five conjugated pyrido[3,4-b]pyrazine-based donor-acceptor copolymers were synthesized by Suzuki and Stille coupling polymerizations, and their photophysical and electrochemical properties and field-effect carrier mobilities were characterized. The copolymer semiconductors include poly(pyrido[3,4-b]pyrazine-alt-9,9-dioctyl-2,7-fluorene) (PPPzF), poly(thiophene-2,5-diyl-alt-2,3-diheptyl-pyrido[3,4-b]pyrazine-5,8-diyl) (PTHPPz), poly(thiophene-2,5-diyl-alt-2,3-didecyl-pyrido[3,4-b]pyrazine-5,8-diyl) (PTDPPz), poly(5,8-bis(4-dodecyl-thiophen-2-yl)-2,3-dioctyl-pyido[3,4-b]pyrazine-a lt-2,5-thiophene) (P3TPPz), and poly(5,8-bis(4-dodecyl-thiophen-2-yl)-2,3-dioctyl-pyrido[3,4-b]pyrazine-alt-9,9-dioctyl-2,7-fluorene) (P2TPPzF). Those soluble in tetrahydrofuran have moderate weight-average molecular weights (19 700-70 400 g/mol). The copolymers have tunable optical absorption bands in the visible region with absorption maxima at 323-588 nm and optical band gaps of 1.75-2.48 eV. Ambipolar redox properties with ionization potentials at 5.20-5.68 eV and electron affinities at 2.86-3.02 eV were observed by cyclic voltammetry. Field-effect hole mobilities of 4.1 x 10(-4)-4.4 x 10(-3) Cm-2/(V S) were observed in PTHPPz, PTDPPz, and P3TPPz copolymers. The combination of ambipolar redox properties and visible region broad absorption bands with fairly high carrier mobilities observed in pyrido[3,4-b]pyrazine-based donor-acceptor copolymer semiconductors make them promising materials for organic electronic and optoelectronic devices.