PURPOSE: To obtain isocyanate-based foams low in thermal conductivity, excellent in resistance to environmental pollution and useful for thermal insulating materials, etc., by reacting a polyisocyanate with a mixture of specific polyethers in the presence of a specific hydrocarbon.

CONSTITUTION: (A) One or more polyisocyanates are reacted with (B) a mixture of (i) one or more nitrogen-free polyethers having two or more hydrogen atoms active towards isocyanates and containing oxyethylene residues and (ii) one or more polyethers having two or more hydrogen atoms active towards isocyanates and containing no oxyethylene groups, (i) and (ii) each having a molecular weight of 400-1,000 in the presence of (C) a hydrocarbon which is vaporizable under the foaming conditions such as n-pentane and, optionally, in the presence of (D) a compound having a molecular weight of 32-399 and, simultaneously, two or more hydrogen atoms reactive towards isocyanates to obtain a desired foam.