Abstract: Liquid unsaturated polyester resins may be thickened and made thixotropic by addition of a colloidal, gelling grade clay such as attapulgite and a quaternary ammonium surfactant under moderate shear. This thixotropic polyester composition is formed without making provision for first chemically reacting clay and surfactant. For enhanced long-term stability a tertiary amine surfactant having an alkyl group and two polyoxyethylene groups attached to the nitrogen may be added to the above composition. For polyester resins having a styrene monomer content greater than 35%, the clay and surfactant are mixed with styrene first, to form a pregel, prior to addition of the polyester resin to be thickened.