Abstract: The invention relates to a process for the prepn of ortho-sulfocarboxylic acids, which comprises: a) in a first reaction step, diazotizing an aromatic o-aminosulfonic acid in the presence of an acid and nitrite in water, in an organic solvent or in a mixture thereof, to an o-diazonium sulfonate, and, b) in a second reaction step, reacting the o-diazonium sulfonate in the presence of a palladium catalyst in water, in an organic solvent or in a mixture thereof, with carbon monoxide, under overpressure, to an aromatic o-sulfocarboxylic acid. The invention also relates to a process for the preparation of sulfonylureas using o-sulfocarboxylic acids as intermediates. Thus, 4-methoxyaniline-2-sulfonic acid was diazotized and treated with CO to give 97% 4-methoxybenzoic acid 2-sulfonic acid.