An electrospray source has been coupled to a reflecting time-of-flight mass spectrometer. The ions enter as a continuous beam in a direction perpendicular to the spectrometer axis and are formed into short bursts of ions with velocities parallel to the axis by electrical pulses applied to injection electrodes. The instrument may be operated either in the linear mode, with ions detected behind the electrostatic mirror, or in the reflecting mode, with ions detected after reflection. In the latter case the time resolution is < or = 8 ns for m/z approximately 600, enabling observation of individual isotopic peaks for masses up to about 4000 u. The sensitivity is adequate to enable measurement of mass spectra for 10 fmol of cytochrome c (approximately 12,000 u). The spectrometer does not limit the range of m/z values, and ions have been observed up to m/z approximately 6000. With proper adjustment of pH and other conditions in the source, the ionization is very soft, enabling injection of weakly bound complexes; these have been accelerated and measured in the spectrometer without observable fragmentation.