The linear optical properties of quaterthiophene (4T) films of about 25 ti thickness were studied, The films were fabricated by vapour deposition of 4T on highly oriented pyrolytic graphite under high vacuum. In the photoluminescence and reflection spectra the vibronic lines are very sharp (FWHM (0-0) less than or equal to 50 cm(-1)) indicating a high degree of structural order in the films. This is supported by the absence of thermal quenching of the luminescence. Comparison with spectra of matrix-isolated 4T reveals very similar vibrational energies, indicating an undistorted structural conformation of 4T in the film, but some additional modes, probably ring deformation modes, are found. In the reflection spectra a splitting of 170 cm(-1) is observed for the first excited electronic level which is tentatively interpreted as Davydov splitting.