Ion mobility (IM) combined with tandem mass spectrometry (MS/MS) has been employed to separate and differentiate between polyether oligomers with the same nominal molecular weights. Poly(ethylene glycol)s with the same nominal mass-to-charge ratio (m/z), but with differing structures, were separated using ion mobility. IM-MS/MS data were able to aid identification of the backbone and end groups of the four individual polyethers in the two sets of isobaric mixtures. The MS/MS data from the resolved oligomers enabled a detailed structural characterization of the polyether mixtures to be completed in one experiment.