It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for the formation of Y2Si2O7 coatings on SiC. The proposed method is validated using an experimental procedure in which the in situ formation of Y2Si2O7 on a commercial SiC‐based fiber is demonstrated. The method involves the deposition of YPO4 on preoxidized fine diameter SiC‐based fibers, and heat treating the coated fibers within a calculated processing window of oxygen partial pressure, temperature, degree of preoxidation, and coating thickness. The results are promising for the development of environmentally resistant interfacial coatings for SiC‐fiber reinforced SiC‐based matrix composites. The proposed and validated approach allows a low‐cost method to obtain continuous hermetic coatings on SiC fibers with interfacial properties adequate for tough composite behavior that resists degradation under turbine engine conditions.