Alpha- and beta-protonated naphthalenes (alpha- and beta-HN+) were investigated by electronic absorption and fluorescence spectroscopies in 6 K neon matrixes using a mass-selected C10H9+ ion beam. The absorption spectra reveal S-1/S-2 <- S-0 transitions with onsets at 502.1 and 396.1 nm for alpha-HN+, and 534.5 and 322.3 nm in the case of beta-HN+. Wavelength-dispersed fluorescence was detected for alpha-HN+, starting at 504.4 nm. Light-induced alpha-H/N+ -> beta-HN+ isomerization was observed upon S-2 <- S-0 excitation of alpha-HN+, whereas beta-HN+ relaxed back into the more stable alpha form either upon excitation to S, or via thermal population of the ground state vibrational levels near the top of the energy barrier between the two isomers. The intramolecular proton transfer leading to the alpha-HN+ <-> beta-HN+ photoisomerization is fully reversible. The observations are explained with the support of theoretical calculations on the ground- and excited states of the isomers, vertical excitation and adiabatic energies, minimum-energy pathways along the relevant reaction coordinates, and conical intersections between the electronic states.