The change of the 2-pyridone-water cluster (2PYH(2)O) structure upon electronic excitation is determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of three different vibronic bands as well as a structural fit based on the rotational constants of eight isotopomers that have been reported by Held and Pratt (J. Am. Chem. Soc., 1993, 115, 9708). A total of 93 emission band intensities were fit, together with the changes of rotational constants of 8 isotopomers. The geometry change upon electronic excitation to the pi pi* state can be described by a strong and unsymmetrical elongation of the hydrogen bonds, a contraction of the OH bond involved in the cyclic cluster arrangement, and an unsymmetrical ring deformation. The resulting geometry changes are interpreted on the basis of ab initio calculations.