Monazite was continuously coated onto Nextel 720 fibers, using! an aqueous precursor and in-line heat treatment at 900 degrees -1300 degreesC. Some experiments were repeated with alumina-doped precursors. Coated fibers were heat-treated for 100 h at 1200 degreesC. Coatings were characterized by optical microscopy, scanning: electron microscopy, and analytical transmission electron micros-copy. Coated-fiber tensile strengths were measured by single-filament tensile tests. The precursors were characterized by X-ray diffractometry, differential thermal analysis/thermogravimetric analysis, and mass spectrometry. Coated-fiber tensile strength was lower for fibers coated at higher deposition temperatures. Heat treatment for 100 h at 1200 degreesC decreased tensile strength further. The coatings were slightly phosphate-rich and enhanced alumina grain growth at the fiber surface, but phosphorus was not detected along the alumina grain boundaries. Fibers with, alumina-doped coatings had higher tensile strengths than those with undoped coatings after heat treatment for 100 h at 1200 degreesC. Alumina added as alpha -alumina particles gave higher strengths than alumina added as colloidal boehmite. Alumina doping slowed! monazite grain growth and formed rough fiber-coating interfaces after 100 It of heat treatment at 1200 degreesC. Possible relationships among precursor characteristics, coating and fiber microstructure development, and strength-degradation mechanisms are discussed in this paper.