N,N'-(Methylenedi-4,1-phenylene)bis(phosphoramidic acid) tetraphenyl ester (PNBTE), a flame retardant with high thermal stability, was synthesized and characterized by elemental analysis (EA), mass spectrometry (MS), infrared spectroscopy (IR) and nuclear magnetic resonance spectroscopy (NMR). The thermal properties of PNBTE were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The solubilities of PNBTE in selected solvents were measured by a gravimetric method. The experimental data were correlated with Wilson, nonrandom two-liquid (NRTL), and universal quasichemical (UNIQUAC) equations. The calculated results showed good agreement with the experimental data. The solubility parameter of PNBTE was estimated by the Scatchard-Hildebrand model; meanwhile, the dissolution enthalpy and entropy, and molar Gibbs energy of PNBTE in the selected solvents are also calculated by the van't Hoff equation.