Single‐phase garnet solid solutions can be synthesized between the composition limits of x=4.18 and x=4.22 in Y2.66Gd0.34FexAl0.677Mn0.09O12 at temperatures between 1340° and 1500°C 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>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 1500°C in O2 increases with increasing Fe content (i.e. as x increases) in the garnet solid solution.