PurposeThe purpose of this paper was to synthesise a thermally expandable microsphere (TEMS) with fast thermal response property and small expansion temperature range, and investigate the factors affecting the expansion properties of the microspheres.Design/methodology/approachA new kind of TEMS with fast thermal response property was synthesised by suspension polymerisation method, using acrylonitrile, ethyl methacrylate and methacrylic acid as the main monomers; Mg(OH)2 as the main dispersing agent; and isooctane or n-hexane or n-pentane as the blowing agent.FindingsThe TEMS possessed the best expansion capacity when encapsulated isooctane and n-hexane were about 18.5 Wt.%. The expansion process of the TEMS could be finished by raising the temperature to 18°C from the expansion onset, much less than the reported 30-50°C. The morphology of the TEMS turned from sphere to irregular concave shape following the content increase of the blowing agent.Originality/valueA new kind of TEMS composed of acrylonitrile/ethyl methacrylate/methacrylic acid as the polymer shell was synthesised. These TEMS showed the fastest thermal response speed reported.