A simple chemical method is described for preparing high density, polycrystalline, magnetic garnets suitable for microwave applications, having the formula 3M2O3·5Fe2O3, where M is a rare earth from Sm to Lu, or yttrium. The effect on the dielectric loss of small deviations from the ideal M:Fe composition was investigated, and it was found that, in contrast to ferrites having the spinel structure a small deficiency of iron gives rise to a considerable increase in the loss. The loss tangents for stoichiometric samples were of the order of 0.002 at 20 mc. The effect of porosity on the ferrimagnetic resonance line width was investigated in several samples of the yttrium garnet and it was found that the observed variation could be accounted for almost entirely by Schlo¨mann's theory of pore broadening, with only a small contribution from anisotropy. The narrowest line observed at room temperature, on a sample having 96% of the theoretical x‐ray density, was 50 oersteds. The trend of the results indicated that a 100% dense sample should have a line width of about one‐half this value.