Relativistic study of xenotime, YPO4, containing atoms thorium and uranium as point defects is performed in the framework of cluster model with using the compound-tunable embedding potential (CTEP) method proposed by us recently [1]. The Y-(PO4)6-Y'22-O'104 cluster for xenotime is considered, in which central part, [Y-(PO4)6] −15 , is the main cluster, whereas outermost 22 atoms of yttrium and 104 atoms of oxygen are treated as its environment and compose electron-free CTEP with the total charge of +15. The P and O atoms of the orthophosphate groups nearest to the central Y atom are treated at all-electron level. The central Y, its substitutes, Th and U, together with environmental Y atoms are described within different versions of the generalized relativistic pseudopotential method [2]. Correctness of our cluster and CTEP models, constructed in the paper, is justified by comparing the Y-O and P-O bond lengths with corresponding periodic structure values of the YPO4 crystal, both experimental and theoretical.Using this cluster model, chemical properties of solitary point defects, X = U, Th, in xenotime are analyzed. It has been shown that the oxidation state +3 is energetically more profitable than +4 not only for thorium but for uranium as well (∆E ≈ 5 eV) despite the notably higher ionic radius of U +3 compared to Y +3 , whereas ionic radii of U +4 and Y +3 are close. This leads to notable local deformation of crystal geometry around the U +3 impurity in xenotime and contradicts to widespread opinion about favorite oxidation state of uranium in such kind of minerals [3].