The composition and lattice parameters of co-precipitated (La0.3Y0.7) orthophosphate were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The results indicate that the as-precipitated powder consists of single-phase (La0.3Y0.7PO4.H2O) rhabdophane nanoparticles. Heat treatment at 950 degrees C caused the decomposition of rhabdophane into a (La1-xYx)PO4 monazite solid solution and YPO4 xenotime. The solid solubility of Y in LaPO4 monazite from 1000 degrees to 1600 degrees C was studied using XRD, TEM, and EDX. The implications of the findings for controlling the coefficient of thermal expansion of the prospective two-phase monazite-xenotime fiber coatings for ceramic composites applications are discussed.