Abstract: A process for the production of naphthylaminehydroxysulphonic acids which comprises simultaneously combining naphthylaminesulphonic acids containing at least two sulphonic acid groups, or the alkali metal salts thereof, and alkali lyes in the form of highly concentrated aqueous solutions at temperatures between 180.degree. and 240.degree. C. and allowing the resulting reaction mixture to react to completion, such that the time required for the metered addition of the components is not more than twice as long as the subsequent reaction time which is necessary under the chosen reaction conditions, while the water content of the reaction melt is between 0.5 and 20%, based on the total melt.