The present invention is directed to a process for producing foamable polyolefin particles comprising adding a mixture of 3 to 15 parts by weight of a blowing agent and 0.5 to 5 parts by weight of a blowing aid to 100 parts by weight of polyolefin particles maintained in a fluid state. The addition of the mixture of blowing agent and blowing aid is conducted under substantially anhydrous conditions, which result in the blowing agent being present in the particles in a gas phase and not a liquid phase. Polyolefin particles comprise polyolefin resin with vinylaromatic resin chemically bonded thereto.