Novel cobalt polyselenidoarsenate [Co(phen)(3)][As2Se2(mu-Se-3)(mu-Ses)] (1; phen = 1,10-phenanthroline) was methanolothermally synthesized by the reaction of CoCl2, As2O3, and Se templated by phen in a CH3OH solvent at 130 degrees C. The same reaction in a H2O solvent yielded cobalt selenidoarsenate [Co(phen)(3)](2)[As8Se14] (2). In 1, the AsSe+ units are alternately joined by the mu-Se-3(2-) and mu-Se-5(2-) bridging ligands to form a novel helical polyselenidoarsenate chain [As2Se2(mu-Se-3)(mu-Se-5)(2-)](infinity). In 2, eight pyramidal AsSe3 units are connected via corner sharing into the new member of the selenidoarsenate aggregate [As8Se14](4) with a condensation grade of 0.571, which represents the first discrete selenidoarsenate(III) with a condensation grade of above 0.50. The octahedral complex [Co(phen)(3)](2+) is formed in situ to act as a countercation in compounds 1 and 2. 1 and 2 exhibit steep absorption band gaps at 2.09 and 2.16 eV, respectively.