A phosphorus flame retardant of 4-(hydroxymethyl)-1-oxido-2,6,7-trioxa-1phosphabicyclo[2.2.2]octane (PEPA) was synthesized by pentaerythritol and phosphorus oxychloride at a certain condition. The structural features of PEPA were investigated with Fourier transform infrared spectroscopy and H-1 NMR methods. The thermal behavior of PEPA was carried out with differential scanning calorimetry and thermogravimetric analysis instruments. A static analytic method was used to measure the solubilities of the PEPA in several organic solvents. The measurements were accurately acted in ethyl acetate, acetonitrile, isopropyl alcohol, ethanol, n-propyl alcohol, acetone, methanol, and chloroform at the temperature range from 292.75 K to 333.45 K. Using the Scatchard-Hildebrand methodology, the activity coefficients of the solute have been calculated, and the partial molar excess enthalpies at infinite dilution were also predicted.