The tunneling splitting associated with double proton transfer recently observed in the dimeric complex 2-pyridone -2-hydroxypyridine is studied theoretically. Direct dynamics calculations based on the approximate instanton method applied to a multidimensional potential-energy surface evaluated at the DFT-B3LYP/6-311++G(d,p) level satisfactorily reproduce the observed tunneling splitting of 0.017 cm(-1) and predict a double deuteron splitting of 6.5 x 10(-1) cm(-1). Comparison with the calculated and observed properties of the excited state indicates that, contrary to an earlier interpretation, this splitting is due to double proton tunneling in the ground rather than the excited state. (C) 2004 Elsevier B.V. All rights reserved.