The Marcus equation was tested, within the framework of ab initio calculations, in the simple proton transfer reaction within the pre-existing hydrogen bonds of complexes containing water and formic acid. Asymmetry was introduced into the transfer potential of (H2O·H·OH2)+ by addition of multiple external ions (e.g. Na+, Cl− and Li+) and point charges or dipoles. The Marcus equation predicts transfer barriers in excellent agreement with quantum calculations, as is also true when the ion approaches the otherwise symmetric protonated formic acid dimer. The situation is less favorable when an external ion introduces a perturbation into an intrinsically asymmetric transfer potential, as in the proton-bound dimer pairing a water with a formic acid molecule. Excellent predictions are obtained when the asymmetry parameter is taken as the effect of the ion itself, damped by a factor of two.