In contrast to the existing cubic langbeinite phosphates and sulphates, orthorhombic langbeinite phosphosilicates of the chemical formula KBaYMP₂SiO₁₂ (M: Zr, Sn), KBaREEZrP₂SiO₁₂ (REE: La, Nd, Sm, Eu, Gd, Dy) and the wasteform KBaY_0.6La_0.1Nd_0.1Sm_0.1Eu_0.1ZrP₂SiO₁₂ have been synthesized by solution method. Powder X-ray diffraction analysis affirmed that the compounds were phase pure and crystallized in an orthorhombic structure with P2₁2₁2₁ space group. Spectral analysis of the wasteform revealed the phase stability, thermal stability and chemical durability of the langbeinite structure. Chemical durability of the powder wasteform has been studied by a dynamic Soxhlet test. Elemental analysis of the leachates showed that the normalized mass losses of barium, zirconium and silicon were in the order of 10⁻³–10⁻² g/m², 10⁻⁵–10⁻⁴ g/m² and 10⁻²–10⁻¹ g/m² respectively. Normalized mass losses of lanthanides, potassium and phosphorous were found to be below the instrumental detection limits.