Using conventional transition state theory, the secondary deuterium kinetic isotope effect (KIE) in the inversion S(N)2 reaction of CH3F and F- is calculated to be small, 0.98 (T = 298 K). This is shown to be the result of a balance among opposing entropy and enthalpy terms. By contrast, KIE in the retention S(N)2 mechanism is calculated to be large (1.5). Accordingly, KIE is a potential observable for discriminating between the two mechanisms. Large KIE'S are also found for the inversion and retention mechanisms of the ion pair reactions between CH3F and LiF. All of the transition structures leading to large KIE'S have a bent FCF angle and an imaginary frequency that is sensitive to deuterium labeling.