The ground state and the excited state of dipyridones are studied by various spectroscopic methods: absorption spectra, fluorescence spectra, time resolved fluorescence measurements, and nanosecond transient absorption spectra. In the system of 3-[(1,6-dihydro-6-oxo-2-pyridinyl)ethynyl]-2(1H)-pyridinone, the absorption and fluorescence spectra are assigned to the keto form in comparison to model compounds. The absorption spectra and the emission decay depend on the concentration in chloroform solution whereas they do not in methanol. This difference can be explained by the formation of a hydrogen-bonded dimer only in chloroform solution. The excited singlet states of the monomer and the dimer have different lifetimes even though they have same keto form. The origin of the fast nonradiative relaxation processes in the dimer system is discussed with the results of transient absorption spectroscopy. As an application of the formation of dimers by hydrogen bonding, the effect of the dimerization on photochemical isomerization is demonstrated in the system of 3-[2-(1,6-dihydro-6-oxo-2-pyridinyl)ehenyl]2(1H)-pyridinone.