A method is described for the determination of the two title compounds that is based on: (a) extraction of the acidified sample with methanol, (b) removal of fats and lipids by partitioning of the extract withn‐hexane, (c) clean‐up on an acidic alumina extraction cartridge, and (d) determination by a post‐HPLC column chemical denitrosation‐thermal energy analyser (TEA) technique or by conventional HPLC‐TEA analysis after derivatization of the compounds with diazomethane. Confirmation was carried out by HPLC‐mass spectrometry of the free acids and also by gas chromatography‐mass spectrometry of the methyl esters. The formation of both 1‐methyl‐2‐nitroso‐1,2,3,4‐tetrahydro‐β‐carboline‐3‐carboxylic acid and 2‐nitroso‐1,2,3,4‐tetrahydro‐β‐carboline‐3‐carboxylic acid in nitrosated samples of several Japanese and Chinese pickled vegetables, one soy sauce, and two cheeses was demonstrated.