The de Boer model for the F centre in CaO has been confirmed by paramagnetic resonance measurements on polycrystalline samples of CaO enriched with ⁴³CaO, the isotropic and anisotropic hyperfine interactions were found to be 9.1 and 1 0 G respectively. The hyperfine lines were also measured for the F centre in neutron-irradiated BaO and isotropic interactions were obtained for ¹³⁵Ba of 75 5 G and for ¹³⁷Ba of 86.3 G, the anisotropic interactions were close to 5 and 6 G respectively. The F-centre hyperfine parameters are compared for the alkaline earth oxides using the point-ion-lattice model of Gourary and Adrian.