Diphenylphosphinic acid (DPPA) was synthesized and characterized by infrared spectroscopy (IR), nuclear magnetic resonance (H-1 NMR), mass spectroscopy (MS), and elemental analysis. The melting point and the enthalpy of fusion of DPPA were measured by a differential scanning calorimeter (DSC), and the thermal stability of DPPA was measured by thermogravimetric analysis (TGA). The solubility data of DPPA in nine solvents were measured and correlated with an empirical equation. The estimated uncertainty of all the solubility values based on error analysis and repeated observations was within 2.0%.