The invention concerns an aqueous slurry comprising a thickener and thermally expandable microspheres comprising a thermoplastic polymer shell and a propellant entrapped therein, said slurry having a pH of at least about 2.5 and comprising a buffer having a pK.sub.a within about .+-.1 pH-unit of the actual pH. The invention further concerns a process for its preparations, use thereof for the production of paper or non-woven, and a process for the production of paper or non-woven.