The optical properties of p-doped polybithiophene films were investigated during the polymerization and discharging process using in situ spectroscopy, while the fluorescence and absorption spectra of the undoped films were ex situ monitored as functions of the film thickness and temperature. The in situ absorption spectra of polybithiophene were observed to depend significantly on the film thickness showing that both the mean effective conjugation length of the polymer chains and the morphology of the films changes during the polymerization. The absorption maximum at 692 nm is assigned to bicationic segments. The similarly structureless absorption spectrum of undoped polybithiophene exhibits a maximum at 480 nm which corresponds to a conjugation length of 7 thiophene units, whereas the red-shifted fluorescence spectrum shows a well-resolved vibrational structure and is attributed to the longest-conjugation length segments (ca. 11 thiophene units) in the film.