Electron spin resonance investigations on aγ-irradiated single crystal of succinic acid have confirmed and extended the findings of Heller and McConnell on the hyperfine couplings in the radical (CO2H)ĊH-CH2(CO2H). Complete coupling tensors, and thegtensor, are given in the table. These confirm that the radical has almost the same orientation as the unchanged molecules. The plane of the free radical carbon is twisted 5° from the original carbon skeleton plane and this leads to appreciable non-equivalence of the hydrogen atoms of the CH2group.