The intrinsic barrier for proton transfer reactions along a preformed hydrogen bond is approximately 1 kcal/mol. The apparent intrinsic barrier for simple apparently bimolecular proton transfers is higher, typically about 5 kcal/mol, because the process involves an intervening water molecule, Analysis of this process by multidimensional Marcus theory shows that the reaction is commonly stepwise, and the apparent barrier arises From the need to form a transient unstable complex of hydronium ion and two basic species or hydroxide ion and two acidic species. A similar analysis leads to predictions of the rate constants for water mediated proton switch reactions.