If the polyisocyanate mixtures according to the invention which contain carbodiimide groups are to be used for the production of foams, water and/or readily volatile organic substances are used as blowing agents, for example acetone, ethyl acetate, halogen substituted alkanes such as methylene chloride, chloroform, ethylidene chloride, vinylidene chloride, monofluorotrichloromethane, chlorodifluor