A prototype electron monochromator (EM) reflectron time-of-flight (TOF) mass spectrometer has been constructed and demonstrated to record resonant electron capture (REC) mass spectra of electron-capturing compounds. The electron energy is ramped from -1.7 to +25 eV at a preset frequency, and the energy spread of the electron beam at 15 nA is 100 meV or better. Ions are orthogonally extracted into the analyzer at a frequency of up to 80 kHz while maintaining an upper m/z-limit of at least 300 and a mass resolving power of approximately 1000. A complete REC mass spectrum, which includes an effective yield versus electron energy curve for each negative ion formed from the compound being analyzed, typically takes several days to produce with a quadrupole or magnetic sector mass spectrometer. With the EM TOF described in this work, three-dimensional negative ion electron capture spectra are recorded in an interval on the order of only 1 s and displayed in real time. This new analytical capability could make it possible to perform GC REC mass spectrometry as well as easier (a) to measure the temperature dependence of REC cross sections, (b) to determine enthalpies of negative ion formation (accurate determination of the enthalpy of ion formation requires knowledge of the translational energy released during a dissociative capture event), and (c) to provide complete thermochemical descriptions of dissociative electron attachment by measuring ion lifetimes.