Proton transfer in the chelated form of malonaldehyde is commonly supposed to occur between two tautomers, across a transition state involving changes of the chemical bonding. We show that this view is in conflict with rotational spectra. The molecule is better thought of as a superposition of indistinguishable and non-separable C-s tautomers and proton tunneling is totally decoupled from the other degrees of freedom. Double minimum potential functions are determined from experiments and ab initio calculations. (c) 2005 Elsevier B.V. All rights reserved.