The invention relates to a polymer comprising quaternary ammonium and/or quaternary phosphonium groups bound to the backbone of the polymer, said quaternary ammonium groups and/or quaternary phosphonium groups being neutralised by counter-ions, wherein the counter-ions consist of the anionic residue of an acid having an aliphatic, aromatic, or alkaryl hydrocarbon group comprising 6 or more carbon atoms. The invention further relates to the preparation of this polymer, its use in anti-fouling compositions, and to anti-fouling compositions comprising the polymer.