Phosphorus-containing flame retardants (FRs) 3,9-dihydroxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro-[5.5]-undecane-3,9-dioxide (dihydroxy-TDUD), 3,9-diphenoxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro-[5.5]-undecane-3,9-dioxide (diphenoxy-TDUD), hydroxymethylphenylphosphinic acid (HMPPA), 2-carboxyethylphenylphosphinic acid (CEPPA), tri(p-methoxyphenyl)phosphine (TMOPP), and tri(p-methoxyphenyl)phosphine oxide (TMOPPO) were characterized by elemental analysis, infrared spectroscopy, (1)H and (31)P nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Using a static analytical method, the solubilities of solutes in selected solvents were measured and correlated with an empirical equation. The estimated uncertainty of all of the solubility values based on error analysis and repeated observations was within 2.0 %.