Arabinonucleosides, Spectrophotometric Titration, Sugar Hydroxyl Dissociation Spectrophotometric titration in the ultraviolet has been employed to determine the pK values for dissociation of the sugar hydroxyls in pyrimidine arabinonucleosides and some of their O′-methyl and O′-ethyl derivatives. The order of dissociation of the sugar hydroxyls in the arabinofuranose ring was 2′-OH > 3′-OH > 5′-OH. The higher acidity (lower pK) of the 2′-OH was interpreted in terms of formation of an intramolecular hydrogen bond of the form 5′-OH···2′-O(-) and the accompanying changes in conformation of the arabinose ring. The various factors affecting the dissociation of specific hydroxyls in some of the O′-alkyl derivatives are discussed in relation to steric, conformational and other effects.