The invention provides novel stable, liquid dyestuff preparations comprising (a) at least one basic dyestuff of the polymethine series present in the form of a salt of an inorganic acid or of an alkylsulphuric acid in which the alkyl radical contains 1 to 4 carbon atoms, (b) at least one water-soluble organic acid and (c) gamma -butyrolactone. The preparations are useful for dyeing polyacrylonitrile, acrylonitrile copolymer, polyester and polypropylene fibres.