Methyl d-pentofuranosides were prepared by Fischer glycosidation of the aldopentoses d-arabinose, d-lyxose, d-ribose, d-xylose, and 2-deoxy- d- erythro-pentose, and oxidized with O 2 over a platinum oxide catalyst to give the corresponding methyl d-pentofuranosiduronic acids. After purification by anion-exchange chromatography, these glycosides were hydrolyzed to give the corresponding d-penturonic acids [ d-arabinuronic acid ( 1), d-lyxuronic acid ( 2), d-riburonic acid ( 3), d-xyluronic acid ( 4), and 2-deoxy- d- erythro-penturonic acid ( 5)] in 80% yield based on the starting pentofuranoside. 1- 13C-Substituted d-aldopentoses were used to prepare d-(I- 13C)penturonic acids. Aqueous solutions of the I- 13C-substituted penturonic acids, studied over a range of pH values by 13C-n.m.r. spectroscopy, were found to contain α- and β-furanoses, acyclic aldehyde and hydrate, and/or hydrated 2,5-lactone. The ratio of d-riburonic acid anomers was most sensitive to solution pH (α/β = 0.49 and 1.2 at pH 1.9 and 4.9, respectively). The values of the 1H and 13C chemical shifts, and 1H 1H, 13C 1H, and 13C 13C spin-coupling constants, were determined by 1H-(300, 500, and 620 MHz) and 13C-(75 MHz)n.m.r. spectroscopy with the aid of 2-D 13C- 1H chemical shift correlation maps, 2-D 1H 1H COSY data, and 13C substitution, and were compared to those determined previously for structurally-related furanose rings. Isomerization of the penturonic acids at pH 5.0 and 50° gave the corresponding 4-pentulosonic acids.