Although a poly(ethylene/propylene glycol) (PEG/PPG) copolymer mixture is far too complex (similar to150 oligomeric formulas) for conventional purification, oligomer ion compositions of <1% abundance can be separated by Fourier transform mass spectrometry and dissociated into sequence-specific fragment ions. Using collisionally activated dissociation (CAD) or other conventional energetic methods, we found that misleading rearrangements are common; however, these are negligible with electron capture dissociation (ECD), consistent with its nonergodic mechanism. Despite the lack of reference compounds, ECD of five oligomers ranging from PEG(1)PPG(18) to PEG(9)PPG(15) shows that similar to80% of their isomers have all PEG units at one end, while CAD gave lower values because of an similar to21% rearrangement loss of internal monomer units. In contrast to the indicated triblock "PEG/PPG/PEG" sample designation of this commercial surfactant, all of these oligomers are found to consist primarily of diblock PEG/PPG structures, so that their termini differ significantly in hydrophobicity, as expected for a surfactant.