7 5 to 12 sodium hydroxide is the preferred alkali metal hydroxide for reasons of cost although koh or lioh are in principle also possible it can be employed both in solid form and in the easily handled aqueous solution of for example 10 to 60 by weight the invention furthermore relates to the use of the above compositions comprising iminodisuccinic acid and the salts of the formula i as bleaching regulators and therefore also to a process for the pretreatment of textile forming natural plant or animal fibers or their mixtures with synthetic fibers or of yarns woven fabrics knitted fabrics or nonwovens of such fibers or their mixtures by bleaching in bleaching liquors which comprise water alkali hydrogen peroxide wetting washing and emulsifying agents and bleaching regulators which comprises employing compositions of the type described above as bleaching regulators the pretreatment process can be carried out in a number of different embodiments all of which are familiar as prior art to the expert pretreatment in a long liquor discontinuous or continuous process cold pad batch process cpb pad steam process pad roll process and others textile forming natural plant fibers for example those of cellulose such as cotton jute linen or regenerated cellulose and animal fibers such as silk and wool and mixtures thereof with synthetics can be bleached according to the invention plant fibers may be mentioned as preferred particularly preferably cotton and mixtures thereof synthetic constituents of the mixture which may be mentioned are synthetic polyamide such as nylon or perlon polyester and polyacrylonitrile in the bleaching liquors to be used according to the invention in addition to water alkali metal hydroxide wetting washing and emulsifying agents and compositions according to the invention as bleaching regulators hydrogen peroxide is employed in an.