The invention relates to thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers comprising from 20 to 80 wt % of acrylonitrile and from 1 to 70 wt % of vinyl ether having only one carbon-to-carbon double bond, the total amount of acrylonitrile and vinyl ether having only one carbon-to-carbon double bond being from 30 to 100 wt % of the ethylenically unsaturated monomers.