Poly(ethylene glycol)s of 4.3, 13, and 23 kDa give mass spectra containing resolved isotopic peaks representing the individual oligomers in each sample. Approximately 5000 isotopic peaks of 47 oligomers in 10 charge states are identified in the 23 kDa spectrum, as well as <0.02%-CH2CH(CH3O- monomer units in the 13 kDa spectrum. As an unexpected advantage of electrospray ionization (ESI), the degree of mass discrimination is much less than that of mass/charge discrimination due to averaging of the values from different charge states. For the determination of molecular weight distributions, geometric and entropy deconvolution methods yield unacceptable artifact peaks and abundance discrimination, respectively; combining their deconvolution attributes with isotopic peak restrictions for the 4.3 kDa polymer yields a distribution similar to that from human data reduction, which is consistent with that from GPC separation. ESI/FTMS spectra of polymers can be measured in minutes and provide far more detailed mass information than that from such conventional techniques.