Abstract: Rigid, closed-cell, flame-resistant polyurethane foams are produced by reacting a polyol mixture with a polyphenyl-polymethylene-polyisocyanate in the present of a catalyst system. The polyol mixture includes: (1) from 25 to 43 wt % flame proofing agent, (2) from 5 to 50 wt % sucrose polyether having an OH number of from 350 to 550, (3) from 0 to 30 wt % amine initiated polyethers having an OH number of from 400 to 800, (4) from 0 to 30 wt % other polyether or polyester having an OH number of from 150 to 600, (5) from 4 to 13 wt % glycerol, (6) from 0.5 to 3 wt % silicone stabilizer and (7) from 0.05 to 1.5 wt % water. The catalyst system is made up of at least two components selected from tertiary amines, alkali metal carboxylates, quaternary ammonium carboxylates and organic tin compounds. The reactants are employed in quantities such that the NCO to OH equivalent ratio is from 1.07 to 1.45 (the isocyanate-index is from 1.07 to 1.45).