A method for producing high-purity derivatized starch which to used in the manufacture of paper and cardboard, a starch raw material is subjected to at least the following steps: purification, slurrying, derivatization, gelatinization, pH adjustment, wherein a slurry of the derivatized starch obtained after the slurrying and derivatization method steps carried out in any sequence is subjected to purification, preferably at an alkaline pH and that the purified and gelatinized derivatized starch obtained after gelatinization is mixed at least once with dilute acid as well as application.