The one-electron spectrum of Th IV was investigated by R.J. Lang (36.1) as soon as 1936, but the ground term 5f²F is due to Klinkenberg and Lang (49.4). Extended observations in the vacuum ultraviolet led Conway and Shalimoff (unp.13) to find excited levels from which a semi-empirical value of the ionization potential has been derived. The two strongest lines of the 5f-6d transition are present in the hollow cathode spectrum recorded by Fourier transform spectrometry (83.9). Klinkenberg reanalyzed his data, added 7 new levels and listed 50 classified lines in (88.8). The g-factors of 9 levels have been determined by Charles (58.1).