The action cross sections for the formation of the cyclobutane dimer and the (6-4) photoproduct of thymine as well as the absorption cross sections of thymine were determined in the wavelength region between 150 and 290 nm. Thymine films sublimed on glass plates were irradiated by monochromatic photons in a vacuum; the induced photoproducts were quantitatively analyzed by high-performance liquid chromatography (HPLC). Under our conditions, two major peaks appeared on the HPLC chromatograms of irradiated samples. The two peaks were identified as being the cis-syn cyclobutane dimer and the (6-4) photoproduct, based on their HPLC retention times, absorption spectra in the effluent, and photochemical reactivity. The fractions of the two photoproducts increased linearly with the fluence at low fluences over the entire wavelength range. Their action cross sections were determined by the slopes of the linear fluence response curve at 10 nm intervals between 150 and 290 nm. The two action spectra showed a similar wavelength dependence and had a maximum at 270 nm as well as two minor peaks at 180 and 220 nm, at which wavelengths the peaks of the absorption spectrum of thymine sublimed on a CaF2 crystal plate appeared. The quantum yields had relatively constant values of around 0.008 for the dimer and 0.013 for the (6-4) photoproduct above 200 nm, decreasing to 0.003 and 0.006, respectively, at 150 nm as the wavelength became shorter.