PURPOSE: To carry out the photo-reduction and fixing of carbon dioxide to oxalic acid, etc. in high efficiency, in one system, by irradiating light to an aqueous medium containing carbon dioxide, 2,2'-bipyridinium dication, a reducing photo- sensitizer, and a reducing agent for the regeneration of the photo-sensitizer.

CONSTITUTION: 2,2'-Bipyridinium dication or 1,10-phenanthrolium dication is irradiated with light in an aqueous medium, preferably in a mixture of water and an organic solvent miscible with water, in the presence of a reducing photo-sensitizer, e.g. tris(2,2'-bipyridine)-ruthenium(II). The dication is converted to the corresponding cation radical by this process, and at the same time, the existing carbon dioxide is reduced with the cation radical and the dication is regenerated. The photo-sensitizer lost it reducing power is reduced and regenerated by the existing reducing agent for the regeneration of the photo-sensitizer (e.g. triethanolamine), and recycled and reused for the reduction of the dication. The carbon dioxide can be converted and fixed to a lower oxyacid, etc. by the photoreduction process.