Phosphorus-containing polyurethanes are synthesized by reacting phosphorus-containing diisocyanates, bis(4-isocyanatophenoxy) phenyl phosphine oxide (BIPPO) and bis(3-isocyanatophenyl) phenyl phosphine oxide (BIPPPO), with various diols. The chemical structures of the obtained monomers and polymers are characterized by IR, H-1- and P-31-NMR spectroscopy. Thermal analysis of the phosphorus-containing polyurethanes reveals that incorporating phosphorus into the polymers increases exothermicity during their decomposition. According to these results, the phosphorus-containing polyurethanes are less thermally stable than conventional polyurethanes. Char yield and LOI measurements demonstrate that incorporating phosphorus into polyurethanes markedly improves their flame retardancy.