Resonance electron capture and thermal electron detachment rate constants have been determined for several low electron affinity (EA) compounds, including anthracene, benzophenone, quinoxaline. These measurements were taken by comparing the molecule of interest with SF(6) using pulsed high pressure mass spectrometry (PHPMS) to evaluate the time profiles for the relevant anions. These measurements were affected by recapture of detached electrons as well as loss of these electrons by diffusion to the walls of the ion source. This dissertation also explains why these low EA molecules are not seen at atmospheric conditions. Using the PHPMS, the reactions of the molecular anions of anthracene, quinazoline, benzophenone, quinoxaline and azulene with oxygen and water have been studied. In the simultaneous presence of oxygen and water, these molecular anions, M, are rapidly destroyed and the ion, (O2 - H2O), is rapidly formed. The high rate with which this transition occurs cannot be explained by the simplest model envisioned that is based on well-known ion molecule reactions. These results can be explained, however, by inclusion into the model of a previously uncharacterized reaction between the molecular ion- oxygen complex, (M-O(2)), and water. The results reported here explain why the molecular anions of compounds that have lower EA's than that of azulene are not readily observed in electron capture ion sources of one atmosphere buffer gas pressure. In addition, it is shown that the reactions characterized here lead to a state of chemical equilibrium between the M and (O2 -H(2)O) ions within the PHPMS ion source from which the EA values of the low-EA compounds can be determined. By this method the electron affinities of anthracene, quinazoline, benzophenone and quinoxaline are found to be 0.54, 0.56, 0.61 and 0.68 eV, respectively.