Boron nitride (BN)-coated aluminoborosilicate (Nextel(TM) 312) fibers, produced via ammonia nitridation, along with 'as-received' and 'desized' fibers, were composited in a silicon oxycarbide (Blackglas(TM)) matrix. The mechanical properties, failure properties, and fiber-matrix interfacial chemistry of the composite were investigated. BN treated fiber composites show a 90% improvement in flexural strength and substantial increases in shear strength (short beam shear and Iosipescu) over the 'as-received' fiber composite. The composite fabricated with 'desized' fibers underwent spontaneous delamination during pyrolization, precluding mechanical testing. X-ray photoelectron spectroscopy of the starting materials and of composite fracture surfaces combined with scanning electron microscopy and energy dispersive X-ray spectroscopy indicate that the locus of failure of the BN-coated fiber composite occurs at the matrix/BN coating interface.