FT-IR and FT-Raman spectra of polycrystalline natural abundance thymine, nine of its isotopic derivatives, and 2-thiothymine (5-methyl-2-thiouracil) are reported from 50 to 3500 cm(-1). The isotopic derivatives are thymine-6-C-13, thymine-alpha-C-13, thymine-N-15(2), thymine-alpha,alpha,alpha,6-d(4), and their respective N-deuterated compounds. The vibrational spectra are assigned using the frequency shifts upon isotopic substitution and are compared to previous assignments, both experimental and ab initio based. Extensive mixing of the vibrations in the region below 1750 cm(-1) complicates the vibrational assignments in this region. However, many of the exocyclic stretching and bending vibrations are assigned with confidence, and many of the couplings can be ascertained with these thymine derivatives. The spectra of the isotopic derivatives described here resolve many of the controversial assignments in the literature and correct some previously misassigned vibrational bands. This represents the most complete experimental vibrational study of thymine to date and provides a useful experimental basis for future theoretical calculations.