Derivatization using 1-phenyl-3-methyl-5-pyrazolone (PMP) was selected among a number of reported methods for labeling carbohydrates, since it gives a quantitative yield, proceeds through a rapid reaction and involves a simple clean-up procedure. Moreover, PMP derivatives provide an increase in sensitivity with ultraviolet and mass spectrometric detection relative to native neutral sugars. Sensitivity studies were carried out using a standard oligosaccharide, tetraglucose. One of the aims of these studies was to determine the minimum amounts of PMPtetraglucose necessary to generate informative full-scan electrospray ionization (ESI) mass spectra and collisioninduced dissociation tandem mass spectra. Another aim was to characterize the fragmentation pattern of PMP derivatives. Quantitative and qualitative studies were also carried out with a typical N-linked oligosaccharide obtained commercially. The PMP-labeled compound underwent directed cleavages which produced fragments containing the reducing end. The native N-linked sugar yielded fragments corresponding to cleavages from both ends of the molecule. Under the same ESI conditions, the N-linked oligosaccharide exhibited more lability, or tendency to fragment, than neutral tetraglucose, in both the derivatized and native forms. Also, PMP labeling was shown to enhance sensitivity in the case of a neutral oligosaccharide, i.e. tetraglucose, whereas the labeling of an Nacetylated oligosaccharide, NGA3, did not yield a noticeable improvement in sensitivity.