Multiple coatings of La-monazite (LaPO4) were applied to Nextel (TM) 610 and 720 oxide fibers using a heterogeneous nucleation and growth coating process with phytic acid solution precursors. The as-processed coatings were found to be largely amorphous. Coated fibers were heat treated at 1200 degrees C for 1-2 h, whereupon the coatings crystallized to porous LaPO4 with thin (50-100 nm) layers of AlPO4 at the fiber-coating interface. Heat-treated fibers exhibited full strength retention, in marked contrast to fibers coated by precipitation using citric acid precursors as in a previous work. The microstructural evolution of the amorphous coatings was studied as a function of temperature. AlPO4 forms at the fiber-coating interface between 700 degrees and 800 degrees C, before high-temperature outgassing of the coating as measured by TGA. NMR spectroscopy was used to study as-precipitated powders and as-processed coatings to gain insights into the structural nature of the materials. A possible explanation for the retention of fiber strength through the coating process is proposed and discussed in light of the experimental evidence.