The invention relates to thermally expandable microspheres comprising a thermoplastic polymer shell and a propellant entrapped therein, wherein said polymer shell is made of a homo- or co-polymer from ethylenically unsaturated monomers comprising more than 85 wt % of nitrile containing monomers and said propellant comprises at least 50 wt % of isooctane. The invention further relates to preparation and use of expandable microspheres, a composition comprising expandable microspheres and articles comprising foamed material containing expanded microspheres.