An ion mobility/mass spectrometry technique has been developed to record mass-resolved ion mobility distributions for multiple ions simultaneously. The approach involves a new instrument that couples an electrospray ion source to an injected-ion drift tube/time-of-flight mass spectrometer. Individual components in a mixture of ions are separated by mobility differences in a drift tube and subsequently dispersed by mass-to-charge ratios in a time-of-flight instrument. Flight times in the mass spectrometer are much shorter than residence times in the drift tube, making it possible to record mass-resolved ion mobilities for all ions simultaneously. The result is a three-dimensional spectrum that contains collision cross section, mass-to-charge, and ion abundance information. The instrument and data acquisition system are described. Examples of combined ion mobility/time-of-flight data are presented for distributions of electrosprayed bradykinin and ubiquitin ions.