In this paper, we report synthesis and characterization of thorium and strontium substituted synthetic monazite solid-solutions of general formula La1-xSrx/2Thx/2PO4 (0 <= x <= 1) using solid-state synthesis route and described the influence of charged coupled (Sr2+, Th4+) substitution in place of La3+ on their thermodynamic stability. High temperature calorimetry has been employed to determine the standard molar enthalpy of formation of La1-xSrx/2Thx/2PO4 (0 <= x <= 1) compounds. The isobaric molar heat capacities of the compounds were measured employing differential scanning calorimetric technique. Based on these experimental data and the available thermodynamic data for the component oxides, thermodynamic properties of the La1-xSrx/2Thx/2PO4 (0 <= x <= 1) have been derived. The negative value of excess enthalpy of formation Delta H-E indicates that the substitution of La by Sr, Th charge couple in the monazite host structure increases their relative stability. Such strong negative interaction parameter is an indication of short-range cation ordering, which found to be more evident at x = 0.3. Based on the experimental results thermodynamic functions for the solid solutions have been generated.