The invention provides a method for the preparation of fluorochlorohydrocarbon(FCHC)-free polyurethane soft (molded) foam materials with low density, by reactingA) liquid polyisocyanate mixtures containing bound urethane groups, containing 22 to 30 weight percent of NCO groups, which in turn are prepared by reacting a mixture of diphenylmethane-diisocyanate and polyphenyl-polymethylene-polyisocyanates (raw MDI) with at least one polyoxypropylene-polyoxyethylene-polyol with a functionality between 2.5 and 3.5, a hydroxyl number between 50 and 90 and a polymerized ethylene oxide group concentration ranging from more than 30 to less than 50 weight percent, relative to the weight of the polymerized ethylene oxide and 1,2 propylene oxide groups, or by reacting of 4,4'diphenylmethane-diisocyanate (MDI) or and MDI-isomeric mixture with the above-named polyoxypropylene-polyoxyethylene-polyol to an NCO-group-containing quasi-prepolymer, and diluting this quasi-prepolymer with raw-MDI, withB) higher-molecular polyhydroxyl compounds and, if appropriateC) low-molecular chain lengthening and/or cross-linkage-promoting materialsD) propellants,E) catalysts as well as if appropriate,F) auxiliary materials and/or additives, as well as the polyisocyanate mixtures modified with urethane groups (A) which are utilizable in accordance with the process of the invention.