A simple ion trap/ion mobility/time-of-flight (TOF) mass spectrometer has been coupled with nanoflow liquid chromatography to examine the feasibility of analyzing mixtures of intact proteins. In this approach proteins are separated using reversed-phase chromatography. As components elute from the column, they are electrosprayed into the gas phase and separated again in a drift tube prior to being dispersed and analyzed in a TOF mass spectrometer. The mobilities of ions through a buffer gas depend upon their collision cross sections and charge states; separation based on these gas-phase parameters provides a new means of simplifying mass spectra and characterizing mixtures. Additionally it is possible to induce dissociation at the exit of the drift tube and examine the fragmentation patterns of specific protein ion charge states and conformations. The approach is demonstrated by examining a simple three-component mixture containing ubiquitin, cytochrome c, and myoglobin and several larger prepared protein mixtures. The potential of this approach for use in proteomic applications is considered.