Fragmentation of thymidine and uridine nucleosides followed by vacuum ultraviolet radiation was compared using time-of-flight (TOF) mass spectroscopy. TOF spectra of 5-methyluridine were also measured in order to investigate the effect of the 5-methyl group of the base and the 2'-hydroxyl group of the sugar moiety on the fragmentation. It was observed that uridine fragments more extensively than thymidine and that the base moiety fragments were more abundant in uridine. It was concluded that the extent and the site of fragmentation are influenced more by the 2'-hydroxyl group of the sugar moiety than the methyl group of the base. It was also observed that the methyl and hydroxyl groups together have a combined effect of stabilizing the nucleoside in respect to thermal dissociation upon vaporization. (C) 2014 Elsevier B.V. All rights reserved.