A phosphorus-containing flame retardant 3,9-dimethyl-3,9-dioxide-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane (PBMP) was characterized by a differential scanning calorimeter (DSC), thermo-gravimetric analysis (TGA), and nuclear magnetic resonance ((1)H NMR and (31)P NMR). Using a static analytical method, the solubilities of PBMP were measured in 10 solvents and correlated with an empirical equation. The estimated uncertainty of all of the solubility values on error analysis and repeated observations was within 2.0 %.