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.