The invention relates to a process for the production of paper or nonwoven fibres comprising the steps of adding thermally expandable microspheres comprising a thermoplastic polymer shell and a propellant entrapped therein to a stock comprising fibres or to a web of fibres, forming paper or nonwoven from the stock or the web, and applying heat to raise the temperature of the microspheres sufficiently for them to expand and thereby increase the bulk of the paper or the nonwoven, wherein said expandable microspheres comprise from about 17 to about 40 wt % propellant and have a volume-average diameter from about 17 to about 35 µm. The invention also concerns certain expandable microspheres and use thereof.