There is disclosed a process for isolating 2-naphthylamine-3,6,8-trisulfonic acid in the form of the alkali or ammonium salt from the sulfonation mixture resulting from the sulfonation of the corresponding naphthylaminemonosulfonic or -disulfonic acid in oleum, which process comprises starting from a sulfonation mixture which contains the naphthylaminetrisulfonic acid in the form of the monopotassium or monoammonium salt, charging said mixture into water or dilute sulfuric acid such that the concentration of the sulfuric acid in the dilute solution or suspension of the sulfonation mixture is 30 to 77% by weight, simultaneously allowing the temperature to rise to 90.degree. to 125.degree. C., subsequently slowly cooling said solution or suspension to a temperature below 40.degree. C., during said cooling adding at least 0.1% by weight, based on naphthylaminetrisulfonic acid, of a dispersant, and isolating the precipitated monopotassium or monoammonium salt of 2-naphthylamine-3,6,8-trisulfonic acid.The naphthylaminetrisulfonic acid isolated in this manner in the form of the monopotassium or monoammonium salt from the sulfonation mixture can be used direct for the manufacture of textile dyes.