AbstractMetastable isomers of stable isotopes may be formed in a nuclear reactor by inelastic neutron scattering and photoexcitation of the stable isotopes, and by radiative neutron capture and (n,2n) reactions on neighboring stable isotopes. The relative importance of these reactions for the production of Ba137m, Cd111m, Se77m, and Sr87mwas evaluated by the irradiation of normal and isotopically enriched samples of the elements in the mixed radiation field of the Union Carbide Research Reactor. Radiative neutron capture is the most important source of Cd111m, Se77mand Sr87min samples of normal isotopic abundance. Inelastic neutron scattering is the prime contributor to the production of Ba137mand is of considerable importance in the production of Cd111m. The (n,2n) reaction appears to be significant only in the production of Ba137m. The formation of these isomers by photoexcitation by the gamma-ray flux associated with the reactor appears to be insignificant when compared with the production by the other three processes. The cross sections determined in these investigations are generally in good agreement with available published values. Reactions for which cross sections were determined and not found in the literature include: Ba137(n,n′)Ba137m,= 0.22b; Cd111(n,n′) Cd111m,= 0.14b; Se77(n,n′)Se77m,= 0.60b;Sr87(n,n′)Sr87m,= 0.12b; Ba138(n,2n)Ba137m,= 2.0mb; and Cd112(n,2n)Cd111m,= 0.35mb. The data indicate that, for short irradiation periods in the fluxes employed in these studies, the metastable isomers Ba137m, Cd111m, Se77mand Sr87mare the most sensitive activation-analysis indicators for these elements and give detection sensitivities in the low nanogram range.