MALDI-TOF mass spectrometry has been used to characterize several hydroxy- and ester-end-functionalized polyether-diols and polyester-diols. Mass spectra have provided information on the number-average molecular weights for low-monol poly(propylene glycol)s that are available through polymerization of propylene oxide in the presence of nonbasic double-metal cyanide complexes. Independent of polydispersity, MALDI-TOF MS proved a powerful method for detecting monofunctional impurities in end-modified telechelics. Although the expected diesters were clearly the main products after the telechelic's OH termini had been reacted with p-nitrobenzoyl chloride, monoester byproducts could be identified when the reaction time for the esterification was too short. Both GPC and NMR analysis were of limited value for detecting monofunctional impurities in ester-modified polyether-diols and polyester-diols. In contrast, MALDI-TOF and electrospray MS could more easily distinguish between diesters and monoesters, and both mass spectrometric techniques gave rise to independent peak series in the mass spectrum provided that the masses of the ions differed by at least 5 Da.