[en] Single-phase garnet solid solutions can be synthesized between the composition limits of x = 4.18 and x = 4.22 in Y₂.₆₆Gd₀.₃₄ Fe/sub x/Al₀.₆₇₇Mn₀.₀₉O₁₂ at temperatures between 1340 and 1500⁰C in O₂. Solid solutions occur only on the Y₂O₃-excess side of the stoichiometric garnet composition. Electromagnetic properties and microstructural features of sintered garnets depend critically on small changes in Fe content in the vicinity of the garnet solid-solution region. An intergranular spinel-type second phase exists for compositions when x is greater than 4.22 and has a deleterious effect on remanent induction and magnetic loss at 3 GHz. The relative density of powder compacts sintered for 16 h at 1500⁰C in O₂ increases with increasing Fe content (i.e., as x increases) in the garnet solid solution