Nextel 610 alumina fibers and alumina-YAG (yttrium-aluminum garnet) matrices were used to make oxide-oxide ceramic matrix composites (CMCs) with and without monazite (LaPO4) fiber-matrix interfaces. Twelve sequential aluminum oxychloride (AlOCl) infiltrations with 1 hour heat treatments at 1100 degrees C and a final 1 hour heat treatment at 1200 degrees C were used for matrix densification. This matrix processing sequence severely degraded CMC mechanical properties. CMC tensile strengths and interlaminar tensile (ILT) strengths were less than 10 MPa and 1 MPa, respectively. Axial fracture of Nextel 610 fibers was observed after ILT testing, highlighting the extreme degradation of fiber strength. Extensive characterization was done to attempt to determine the responsible degradation mechanisms. Changes in Nextel 610 fiber microstructure after CMC processing were characterized by optical microscopy, SEM, and extensively by TEM. In AlOCl degraded fibers, grain boundaries near the fiber surface were wetted with a glass that contained Y2O3/SiO2 or Y2O3/La2O3/P2O5/SiO2, and near-surface pores were partially filled with Al2O3. This glass must also contain some Al2O3 and initially some chlorine. AlOCl decomposition products were predicted using the FactSage((R)) Thermochemical code, and were characterized by mass spectrometry. Effects of AlOCl precursors on monazite coated and uncoated Nextel 610 fibers tow and filament strength were evaluated. A mechanism for the severe degradation of the oxide-oxide CMCs and Nextel 610 fibers that involves subcritical crack growth promoted by release of chlorine containing species during breakdown of intergranular glasses in an anhydrous environment is proposed.