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, from 20 to 80 wt % of monomers selected from the group consisting of esters of acrylic acid, from 0 to 10 wt % of methacrylonitrile, from 0 to 40 wt % of monomers selected from the group consisting of esters of methacrylic acid, the total amount of acrylonitrile and esters of acrylic acid constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising at least one of methane, ethane, propane, isobutane, n-butane and isopentane. The invention further relates to the production and use of the microspheres.