The radical trapped in x-irradiated monofluoracetamide is shown to be CHF CONH2by electron spin resonance studies. The principal values of the hydrogen hyperfine coupling are -96, -63 and -31 Mc/s and of the fluorine coupling are +530, -11 and -45 Mc/s. The relative signs of the principal couplings were established, using a spectrometer operating at 35 000 Mc/s which is briefly described. The differences in spectra between measurements at 9000 Mc/s and at 35 000 Mc/s are indicated. Finally the couplings are interpreted and shown to be consistent with Hückel parameters for fluorine with (αF-αC) = 2·4βC-F.