In this study, gamma irradiated NH4HCOO single crystal and CsHCOO powders were investigated at room temperature. The species, produced by gamma irradiation was found to be CO2-, in NH4HCOO, and the principal values of g, and the hyperfine coupling tensors of the unpaired electron with the C-13 nucleus were obtained. In CsHCOO powders, the species was attributed to the CO3- radical, and a superhyperfine interaction with the Cs-55 nucleus, of nuclear spin I = 7/2, has been observed, and this has been explained by the interaction of the unpaired electron with a nearby Cs-55 nucleus. The superhyperfine constant was found to be approximate to 1 mT.