We have measured the rate constants for the reactions of O-2(.-) and several related anions and radical anions with alkyl halides. The results suggest that O-2(.-) reacts with alkyl halides more rapidly than anticipated based on anion basicities as well as on results from ab initio calculations. In addition. deuterated neutrals were used and the kinetic isotope effects (KIE) determined. These values were also calculated by using ab initio calculations and transition state theory. The agreement between experiment and theory is excellent, except for O-2(.-) indicating that this reaction proceeds by a different reaction mechanism. We discuss competition between electron transfer and substitution mechanisms to explain the observed remarkable nucleophilicity of O-2(.-). A detailed analysis of the different contributions to the calculated KIEs shows that the total value is primarily determined by the C-H stretching and the CH3 out-of-plane bending modes. The former factor is very inverse (k(H)/k(D) < 1), and it does not change significantly for different reactions. However, the bending factor changes markedly from one reaction to another and therefore becomes the factor that determines how the total KIEs change for different reactions. As a result, the KIEs are related to the crowdedness of the transition states. This effect is discussed in detail with respect to the conventional looseness interpretation.