The intrinsic barrier in the methyl transfer reaction $X^- + CH_3X ightleftharpoons XCH_3 + X^-$ has been shown to vary quadratically with the C-X distance d; linear dependence of the intrinsic barrier on the deformation energies or methyl cation affinities of the substrate, $CH_3X$, should therefore hold only approximately in a narrow range of structural variations in X.