The invention relates to a process for producing foams based on polyisocyanates by reacting polyisocyanates, compounds having at least two hydrogen atoms reactive towards isocyanate groups, blowing agents and, if appropriate, further additives. In this process, a blowing agent is used which is composed to the extent of at least 10 mol % of one or more fluorinated ethers of the formulaC.sub.a H.sub.b F.sub.c --O--C.sub.d H.sub.e F.sub.fwhere______________________________________ a = 1-6 d = 1-2 b = 1-12 e = 0-5 c = 1-12 f = 0-5. ______________________________________