A composition comprising a particulate dispersion of from 50% to 98.9% by weight of a siloxane-polyoxyalkylene copolymer surfactant, from 1% to 49.9% by weight of a nonionic organic surfactant other than the siloxane-polyoxyalkylene copolymer surfactant and from 0.1% to 25% by weight, based on the total weight of the composition, of a particulate material having a particle size of less than 20 microns, is described. A method of preparing such a composition, and polyurethane and polyisocyanurate foams containing such a composition are also described. The nonionic organic surfactant is preferably an ethoxylated nonylphenol. The particulate material is preferably a hydrophobic precipitated silica. The composition of the invention is useful as an additive in the preparation of rigid polyurethane and polyisocyanurate foams.