In this study a Th-bearing monazite from a Brazil beach sand, a low Th monazite from a Malawi carbonatite, and a xenotime from a pegmatite in northern Pakistan were experimentally metasomatised in a series of common metamorphic and igneous fluids at 600°C/500 MPa and 900°C/1000 MPa. Fluids included H2O, NaCl, and KCl brines, CaF2 + H2O, 1m and 2m HCl, 1m and 2m H2SO4, 1m NaOH, and Na2Si2O5 + H2O. The monazite show a variety of responses to the fluids ranging from no reaction (KCl + H2O) to small compositional changes and partial replacement of the monazite grain rim by Th-enriched monazite in the NaOH and (Na2Si2O5 + H2O) experiments respectively. The other acid and brine fluids induced varying degrees of partial dissolution in the monazite and xenotime, but no compositional alteration. Partial replacement of monazite grain rims by Th-enriched monazite occurred only in the alkaline fluids as the result of a coupled dissolution-reprecipitation process.