[en] In order to improve the preparation of densified samples of solid solutions of β-PDTU (β-Th_4-xU_x(PO₄)₄P₂O₇, an original synthesis way based on the initial precipitation of crystallized solid precursors has been developed. These last ones are prepared from a mixture of solutions containing the cations in hydrochloric and phosphoric acid medium. The gelatinous phase initially formed is converted into a crystallized precipitate by heat treatment in closed system, either on a sand bank (160 C, 1 day) or in a calorimetric bomb (150 C, 1 month). Either be the experimental conditions and the considered compositions, the analysis by powder X-ray diffraction has revealed the obtained phase as being the phosphate-hydrogeno-phosphate of thorium and uranium (IV) mono-hydrated (Th_2-xU_x/2(PO₄)₂(HPO₄), H₂O: PHPTUH). The elementary analyses (PIXE, MASE) have, moreover, confirmed that the PHPTUH solid solutions was systematically mono-phased. Nevertheless, the different steps leading to the transformation of PHPTUH to β-PDTU during a high temperature heat treatment have been revealed by several characterization techniques (DTA, TGA, IR and μ-Raman spectroscopies...). These techniques have revealed a low temperature form of PDTU, called α-PDTU, unknown until now. The solid solutions of mono-phased β-PDTU are obtained after a high temperature treatment of the solid solutions of PHPTUH for molar percentages of uranium inferior to 70%. The optimal densification conditions of these solid solutions have been determined by dilatometric analyses. After pelletizing at ambient temperature by uniaxial pressing (100-300 MPa), the compacts present a maximal hammering for a heat treatment of 5 to 10 hours at 1250 C. The corresponding density reaches then 95 to 98 % of the calculated value equivalent to a low global porosity (2 to 5%). The elementary analyses carried out confirm that all the solids thus prepared are homogeneous and mono-phased. At last, the alteration resistance of the dense samples of β-PDTU by aqueous solutions has been estimated during leaching tests. Either be the used experimental conditions (temperature, pH, nature of the electrolyte, change rate...), different behaviours have been observed for the different cations: the uranium, on account of its oxidation to uranyl ion, is preferentially released in the leachate while the thorium precipitates rapidly under the form of phosphated neo-formed phase(s). On the other hand, the dissolution velocities appear always weaker compared to those obtained for other studied materials (normalized leaching rate, R_L(U), determined from the uranium (VI) concentration released in solution of about 1.1*10^-4 g.m^-2.j^-1 and of 2.8*10^-6 g.m^-2.j^-1 respectively in HNO₃ 10^-1 M and 10^-4 M, at 90 C. The surface analyse of the altered samples by XRD in grazing incidence and by MASE has allowed to identify the neo-formed phase from PHPTUH strongly impoverished in uranium. With all these results, it has been possible to propose a dissolution mechanism for this material and to reveal too a strong decrease of the actinides release due to the formation of neo-formed phases at the surface of the altered solids. (O.M.)