Pure 99Mo (CO)6 was produced free of other reaction products by nuclear fission in a mixture of Cr(CO)6 and U3O8 according to the reaction Cr(CO)6+U (n,f) 99Mo → 99Mo (CO)6. The relative yield ranged up to 60% and was equal to the yield we had found formerly for the production of 103Ru (C5H5) 2 by the same method.