The introduction of small amounts of iodine in ferrimagnetic single crystals causes an extensive modification of the magnetic properties. We observe from microwave measurements made on crystal spheres at temperatures between 80°K and 300°K a large reduction both in the resonance field and in the ferrimagnetic anisotropy energy. Thus forNiFe2O4and for YIG, −K1/Ms at 80°K decreases from 340 to 12 Oe and from 45 to 15 Oe, respectively. The large reduction of anisotropy can be explained on the basis of a single ion anisotropy model. The new properties (very weak magnetic anisotropy and low resonance field) are very favourable for microwave ferrite devices.