Tautomerism of cyanopyridone (CPy) has been studied in both the ground (S-0) and first singlet excited (S-1) electronic states by means of steady-state and time resolved optical spectroscopy as well as time-dependent density functional theory (TDDFT) calculations. The enol (E)-to-keto (K) tautomerization reactions occur in both the ground and excited states of the neutral structures in water. Gas-phase calculations for the K and E tautomers support that both have similar energies in the ground state. Calculations of the S-1 states of each tautomer suggest the energy difference between the tautomers is increased in the excited state, favoring the K tautomer. (C) 2014 Elsevier B.V. All rights reserved.