The standard molar enthalpy of formation of thorium phosphate diphosphate, β-Th4(PO4)4P2O7(cr) hasbeen determined sing an isoperibol solution calorimeter, a differential scanning calorimeter and phasetransition data of high temperature calorimeter. The enthalpy of precipitation of thorium phosphate-hydrogenphosphate hydrate, Th2(PO4)2(HPO4)·H2O(cr), was measured at 298.15 K using an isoperibol solution calorimeter. Heat capacities of α-Th4(PO4)4P2O7(cr) and β-Th4(PO4)4P2O7(cr) were measured using a Differential Scanning Calorimeter. Combining these experimental data, and other auxiliary datafrom the literature, thermo-chemical reaction scheme was devised to calculate the standard molarenthalpy of formation of β-Th4(PO4)4P2O7(cr) to be {−10565.5 ± 13.6} kJ mol−1. The thermodynamic functions for β-Th4(PO4)4P2O7(cr) have been computed using ΔfH°m(298.15 K), C°p,m(T) and literaturedata. The Gibbs phase diagram and predominant area diagrams for Th-P-O system have been computed using the thermodynamic information of the various phase present in Th-P, P-O and ThO2-P2O5 systems.EH–pH diagram of Th-P-H2O system has been computed to determine stability of Th4P6O23(cr) in groundwater.