The Thorium distribution between monazite and xenotime has been determined experimentally using the coupled substitution Th + Si ⇔ REE + P. Experiments have been conducted in standard cold seal hydrothermal and internally heated pressure vessels at 200 MPa in the range of 600–1100°C. Starting mixtures were prepared from gels composed of equal amounts of CePO4 and YPO4 with addition of 10, 20 and 50 mol% ThSiO4. The grain sizes of the run products were in the range of a few microns. Analytical electron microscopy (AEM) methods were applied to obtain reliable chemical compositions of the reaction products. Lattice parameters of run products were determined using Rietveld analysis. For runs with 10 and 20 mol% ThSiO4 component in the bulk the ThSiO4 component distributes almost exclusively into monazite at all temperatures. The amount of the YPO4 component in monazite increases relative to the Th-free system if significant amounts of ThSiO4 are present within the structure. ThSiO4 favours incorporation of YPO4 resulting in a shift of the monazite limb and the shrinkage of the monazite-xenotime miscibility gap in the CePO4-YPO4-ThSiO4 ternary diagram. Thermometric calculations based on monazite-xenotime equilibria must be corrected for this effect. For runs with 50 mol% ThSiO4 in the bulk, thorite formed as an additional phase at 600 to 900°C but was absent at higher temperatures. At high XThSiO4bulk and low T, the system is three-phase. The three-phase stability field strongly shrinks with increasing temperature. A tentative phase diagram of the ternary system CePO4-YPO4-ThSiO4 is proposed and discussed in the light of monazite-xenotime-thorite-bearing assemblages in natural rocks.