Through the analysis of sufficiently fast identity exchange reactions, the Marcus theory additivity postulate has been tested and verified for two sets of gas-phase SN2 reactions : X- + RCH2Y --> XCH2R + Y- (where X and Y = Cl and/or Br and R = CN, C6H5). Statistical RRKM theory, within the microcanonical variational transition state (muVTS) approximation, is used to interpret the experimental kinetic data for each reaction to estimate the activation energies relative to separated reactants. Complexation energies, determined experimentally, are used in conjunction with the data from the RRKM analysis to obtain potential surface energetics. The Marcus expression is then used to determine whether the intrinsic component of the activation energy for the cross reaction is the mean of the activation energies for the two corresponding identity exchange reactions. Good agreement is found for R = C6H5 and CN.