Phase formation in the mixed sulfate-selenate Np(VI)-bearing aqueous system has been investigated. The obtained crystalline compounds, Cs ₂ [(NpO ₂ ) ₂ (TO ₄ ) ₃ ] (T = S, Se), crystallize in the tetragonal system, space group P4 2 ₁ m, with a = 9.5737(3)-9.817(3) Å, c = 8.0824(4)-8.111(3) Å, and V = 740.79(6)-781.8(6) Å ³ , and have been chemically characterized. Single-crystal X-ray diffraction experiments at various temperatures were used to define the thermal behavior of the crystal structures relative to the S/Se ratio. The thermal behavior of mixed sulfate-selenate and pure selenate compounds is anisotropic, with the highest thermal expansion in the direction perpendicular to the sheets of Np-bearing polyhedra, while the structure of the pure sulfate compound experiences almost isotropic thermal expansion with slightly higher expansion within the layer, which is explained by the influence of stronger Cs-O bonding. High-temperature drop solution calorimetry was used to derive the enthalpies of formation of the Cs ⁺ -bearing neptunyl oxysalts. Within this family of isotypic structures, cesium neptunyl sulfate has a more negative enthalpy of formation in comparison to the selenate compound.