The invention relates to a process for the manufacture of thermally expandable thermoplastic microspheres. The process comprises, providing a mixture of monomeric materials suitable for polymerisation to form a thermoplastic polymer and at least one blowing agent, providing to the mixture a colloidal silica that is surface-modified with at least hydrophobic organosilane groups and forming an emulsion. A polymerisation is performed to form the thermally expandable thermoplastic microspheres. The invention further relates to thermally expandable thermoplastic micro spheres, expanded micro spheres and their use in the manufacture of products.