AbstractA flame retardant composition was prepared from phosphorus‐melamine‐urea‐formaldehyde (PMUF) reaction. The flame retardancy of rigid polyurethane foam (RPUF) was studied by impregnating it with various concentrations of PMUF. Optimum impregnation time, retention and density of RPUF samples with PMUF were also studied. The morphology, elemental analysis, and thermal stability of RPUF and RPUF‐PMUF samples were studied with scanning electron microscopy (SEM), CHNS analyzer, and energy dispersive X‐rays (EDX) and, thermogravimetric analysis (TGA), derivative thermogravimetric (DTG), differential thermal analysis (DTA), respectively. The deposition of PMUF on the cell walls and surfaces was exhibited by decreased cell size from 357 to 285 (±5) μm. CHNS and EDX studies show the introduction of phosphorus and nitrogen into the RPUF‐PMUF samples. The degradation of RPUF and RPUF‐PMUF samples was occurred through two and three stages in nitrogen and air, respectively. RPUF‐PMUF samples are decomposed comparatively at lower temperature than RPUF which is resulted into large amount of high temperature stable char residue. This char residue acts as insulating blanket and protects RPUF‐PMUF from fire. Fire performance and smoke density of RPUF and RPUF‐PMUF samples were investigated with BS: 4735, ASTM D2863 and ASTM E662, respectively. RPUF‐PMUF samples demonstrate reduced extent burnt from 125 to 27 mm, burning rate 2.23–0.44 mm/s and percent mass loss (PML) from 100 to 8.82%. Oxygen index was also increased from 17.8 to 23.8. Dm results obtained during smoke density tests show that RPUF‐PMUF generates less and delayed smoke than RPUF under flaming and nonflaming mode. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008