Abstract: A process for the preparation of an o-naphthoquinonediazide sulfonic acid ester is disclosed. Esterification of an o-naphthoquinonediazide sulfonic acid halide with a mono- or polyvalent phenolic compound in a solvent is performed in the presence of ammonia, of ammonium salts of weak acids or of aliphatic derivatives of ammonia having 1 to 3 carbon atoms, at a pH within the range from about 1.5 to about 8.5 and a temperature within the range from about 15.degree. C. to about 40.degree. C. The esters of o-naphthoquinonediazide sulfonic acid which are obtained by this process contain only small amounts of metal ions and can be used in photosensitive mixtures satisfying high requirements of the microelectronics industry.