Although it is well known that arginine side chains of proteins become extensively modified in the course of the advanced Maillard reaction, to date only few possible arginine derivatives are known. Recently, N-delta-[5-(3-hydroxypropyl)-4-oxo-imidazolon-2-yl]-L- ornithine (PIO) was identified as a new arginine derivative, formed exclusively from the side-chain of peptide-bound arginine and degradation products of oligosaccharides with 1,4-glycosidic linkages. In the present paper, a previously unknown C5 dicarbonyl compound, namely 3,4-dideoxypentosulose (3,4-DDPS), was isolated by means of semi-preparative RP-HPLC as the corresponding chinoxaline after heating lactose followed by trapping the 1,2-dicarbonyl compound with o-phenylenediamine. Identification was achieved using LC-MS as well as H-1- and C-13-NMR. Synthesis of 3,4-DDPS as the corresponding 2-hydroxy- dihydro-pyran-3-one proved the structure analysis. During heating of N-alpha-hippuryl arginine (Hip-Arg) with 2-hydroxy-dihydro-pyran-3-one, Hip-PIO was formed as the only arginine derivative. The formation of 3,4-DDPS from oligosaccharides with 1,4-glycosidic linkages follows a new and quantitatively important pathway of carbohydrate degradation in foods.