The experimental technique of ultraviolet photoelectron spectroscopy has been applied to a number of merocyanine dyes related in chemical structure. Additionally, polarographic and optical measurements were made on these dyes in solution, and Hu¨ckel molecular orbital calculations were carried out for isolated dye molecules. The observed photoionization threshold decreases with increasing delocalization of &pgr; electrons in the compound. With the use of absorption data, we constructed energy band diagrams for the dyes, assuming that these organic materials exist as molecular solids. Our assumption was supported by the observation that their ionization thresholds correlate well with the polarographic and the computational results. Photoelectron spectroscopy is deemed to provide a powerful method for studying dyes in practical forms.