It is generally assumed that the formation of low barrier hydrogen bonds (LBHBs) requires identical proton affinities (Delta PA = 0) for the bases linked by the LBHB. This is generally true for LBHBs between identical interface atoms, while values of Delta PA not equal 0 have been reported for LBHBs connecting chemically different atoms. The inherent asymmetry in heteronuclear LBHBs requires Delta PA not equal 0 to compensate for the asymmetry in the potential energy surface of the LBHB. A PA mismatch between the bases in joined by a heteronuclear LBHB is therefore the standard case and not an exception from the rule. (c) 2006 Published by Elsevier B.V.