The present invention relates to a cyclic continuous process and composition for the removal of NO and SO.sub.2 from a variety of fluid streams. A fluid stream containing NO and SO.sub.2 is contacted with an aqueous solution of water-soluble organic polymeric chelate containing a polyvalent metal, e.g., Fe (II). The NO is catalytically absorbed and in the presence of SO.sub.2, the imidodisulfonate is formed. Optionally, a reducing agent, such as sodium hydrogen sulfide, is added to maintain the polyvalent metal in the reduced state. The process next includes removal of the imidodisulfonate formed. The separation of water and low molecular weight materials and products, e.g. molecular weight below 500, usually occurs by ultrafiltration or dialysis, with recycle and re-use of the polyvalent metal chelate.