Equilibria of transfer of pyridine and benzene derivatives, from the vapor phase to dilute aqueous solution, are enhanced by the introduction of exocyclic amino and hydroxyl substituents. Much larger increases are associated with the introduction of imino and keto substituents. Purine derivatives exhibit comparable behavior. These observations are discussed in relation to group transfer potentials, the observed affinities of nucleic acid bases for the active sites of proteins, environmental influences on the occurrence of rate tautomers that lead to errors in base pairing, and hypotheses concerning the origins of the genetic code.