The S-0-S-1 transition of 1- and 2-perfluoronaphthol (1-FN and 2-FN) has been studied by fluorescence excitation and dispersed emission in a continuous supersonic expansion. Each compound gives rise at the origin to two dissimilar excitation systems which can be distinguished by their emission region, While the higher energy systems exhibit both in absorption and in emission the characteristic features of unsubstituted perfluoronaphthalene, the low-energy systems present a different complex pattern typical of a strongly distorted excited state. The dispersed fluorescence from 1-FN shows a resolved resonant structure for both systems, but the emission extends largely toward the red for the excitation of the low-energy system excitation. The dispersed emission from 2-FN low-energy system is totally diffuse and red shifted, with a maximum at about 4500 cm(-1) from the excitation. The two absorption spectra have been tentatively attributed to the presence of the ground state cis and trans rotamers corresponding to a different orientation of the OH group with respect to the naphthalene frame and involving different weak hydrogen bonding of OH with the neighboring fluorine atom. The spectroscopic properties of each rotamer have been shown to be strongly sensitive to the position of the weak internal hydrogen bond.