Polyaromatic polyisocyanates are modified by reaction with 0.1 to 10 weight %, preferably 2 to 7 weight % (based on polyisocyanate) of a polyether alcohol of molecular weight 206 to 948, obtainable by the reaction of alkylene oxides with C.sub.6 -C.sub.30, preferably C.sub.8 -C.sub.20, hydrocarbons containing 1 to 3 active H atoms, and are used for the production of rigid polyurethane foamed materials.