A mixture of peptides from a complete tryptic digest of ubiquitin has been analyzed by ion mobility/time-of-flight mass spectrometry techniques. All components of the mixture were electrosprayed and ions were separated in the gas phase based on differences in their mobilities through helium before being dispersed into a time-of-flight mass spectrometer for mass-to-charge analysis. The data show that ions separate into families primarily according to differences in their charge states and, to a lesser extent, differences in conformation. This approach reduces spectral congestion typically associated with electrosprayed mixtures and provides charge assignments for mass-to-charge ratio data. Gas-phase separations of ions appear to provide a new physical basis for characterizing components of biological mixtures.