This invention relates to a process for the preparation of an isotropic rigid foam comprising the step of reacting: (A) an organic polyisocyanate component selected from the group consisting of: (1) a polymethylene poly(phenylisocyanate) and (2) an NCO-terminated prepolymer, and comprising the reaction product of: (a) a polyester comprising the reaction product of (i) pure ortho-phthalic acid or phthalic anhydride and (ii) a glycol and (b) a polymethylene poly(phenylisocyanate); with (B) an isocyanate-reactive component comprising: (1) a liquid OH-terminated prepolymer having a viscosity of more than about 100,000 mPa·s at 25° C., and which comprises the reaction product of: (a) an organic aromatic polyisocyanate component; and (b) an amine initiated polyether polyol component; (2) at least one polyether polyol and optionally (3) at least one aromatic polyester polyol; in the presence of: (C) at least one catalyst; and (D) at least one blowing agent; wherein the relative amounts of components present is such that the Isocyanate Index is from 90 to 170. This invention also relates to the isotropic rigid foams prepared thereby.