Copolymerization of cyclohexene oxide with carbon dioxide was investigated by using chromium complexes with salalen ligands which are reduced analogues of salen ones (salen-H-2 N,N'-disalicylidene-1,2-ethylenediamine). Although the use of (salalen)CrCl complexes alone without additives was not effective for the copolymerization, a mixture of (salalcn)CrCl complexes and onium salts catalyzed the copolymerization with relatively high catalytic activity. Particularly, the present catalyst system showed the highest catalytic activity under atmospheric pressure of CO2 among those ever reported. In general, the obtained copolymers consisted of almost perfectly alternating structure. Structural analysis of the copolymers by MALDI-TOF mass spectrometry revealed that chloride initiated the copolymerization. In addition, concomitant water was found to work as a chain transfer reagent and a bifunctional initiator.