Absolute dissociative ionization cross-sections from threshold to 200 eV have been measured using Fourier transform mass spectrometry (FTMS). In the production of positive ions by electron impact ionization, 13 ions are detected, including the parent ion CH3NO2+ and the four most important fragment ions CH3NO+, NO2+, NO+, and CH3+. Kinetic energies of some selected positive ions are determined using the technique of trapping potential dependency study in FTMS experiments. In the production of negative ions by dissociative electron attachment to nitromethane, three anions were detected: CH2NO2-, NO2-, and CN-. The high sensitivity of the FTMS permits the subsequent time-resolved reactions of these anions to be studied at very low pressure.