In this study, a natural Th-absent xenotime [(Y+HREE)PO4], is enriched in specific areas with respect to Th + Si utilizing a series of alkali-bearing fluids that included 2 N NaOH, 2 N KOH, Na2Si2O5 + H2O, and NaF + H2O, in addition to ThO2 and SiO2. Charge and fluid were sealed in Au capsules and placed in the piston-cylinder apparatus (CaF2 assemblies) at 1000 MPa and 900 °C (8 to 25 days) or in cold-seal autoclaves on a hydrothermal line at 500 MPa and 600 °C (23 days). BSE imaging, EMP analysis, and TEM indicate that a fraction of the xenotime grains in the 2 N KOH, Na2Si2O5 + H2O, and NaF + H2O experiments have altered areas enriched in Th + Si. No reaction was observed in the 2 N NaOH experiments. The altered areas occur as a series of curvilinear intergrowths with sharp compositional boundaries that extend from the edge of the xenotime grain into the interior. Formation of these Th + Si enriched areas is interpreted as a consequence of fluid-mediated coupled dissolution-reprecipitation.