Abstract: A method for photochemical separation or enrichment of isotopes of carbon-13 and oxygen-18 employing 123.58 nm resonance radiation of Kr of selected band-width and degrees of self-absorption in order to excite selectively to the A.sup.1 .pi.v'=13 state, .sup.13 C.sup.16 O, .sup.12 C.sup.18 O, or both .sup.13 C.sup.16 O and .sup.12 C.sup.18 O simultaneously, in a mixture of isotopic CO molecules where the .sup.12 C.sup.16 O isotopic molecule is in a large excess. The electronically excited isotopic CO molecules react with a second reactant to yield isotope enriched final products which can be separated; for example, ground state CO as second reactant yields CO.sub.2 and C.sub.3 O.sub.2 products.