The intramolecular proton-transfer process in malonaldehyde is studied by ab initio methods in its ground electronic state and in its excited r - r* triplet state. The transfer barrier is considerably higher in T1, which is attributed in part to the virtual disappearance of the hydrogen-bonding interaction present in the ground state. The weakening of this H bond in the excited state is associated with a less acidic protondonor OH group and less basic acceptor oxygen. The r - r* excitation adds antibonding character to the C=O bond. which causes it to elongate. The ensuing reduction in its bond dipole help to weaken the intramolecular H bond.