Polyisocyanate-based foams having a uniformly fine cell size are prepared by reacting a polyisocyanate with an isocyanate-reactive compound in the presence of a blowing agent mixture comprising a hydrocarbon component which has four or more carbon atoms and a polyfluorocarbon component which has a boiling point of from about 283 K. to about 393 K. at 760 mm Hg and wherein the boiling point:molecular weight ratio of said polyfluorocarbon component is less than or equal to 1.7. The hydrocarbon component is present in from about 5 to about 95 mole percent and the polyfluorocarbon in from about 95 to about 5 mole percent based on total amounts of hydrocarbon and polyfluorocarbon.