The optical characteristics in the vacuum ultraviolet region were studied using the molecular orbital method. A model molecule, p-ethylphenol, was used to represent poly(p-hydroxystyrene), and its spectrum was calculated by the semiempirical molecular orbital calculations. The theoretical results were compared with the experimental ones. The wavelengths of the absorption peaks showed good agreement in both results, though the intensities of the absorption did not agree. The fluorination of p-ethylphenol was investigated and it was found that the increase in the number of fluorine atoms became more transparent in the compound. The cause was explained by two factors. One was the redshift of the pi-pi* electronic transition of the aromatic molecules, of which the oscillator strength did not depend upon the fluorination number. The other was the decrease in the oscillator strength of the peak at 157 nm, of which the wavelength did not significantly shift.