The emission spectrum of Pa IV has not been observed. The systematic analysis of optical spectra and electronic structure of actinide ions in compounds and in solutions led to estimate 5f^N energy parameters which are affected by ligand-field interaction (84.8; unp.11). These parameters were used to predict the energy levels of the ground configuration 5f². Although the level structure of the gaseous free ion Pa³⁺ may be different, the theoretical energies reported below are data of interest for unraveling the spark spectra of protactinium when available. The energy of the lowest levels has been predicted for 4 excited configurations by Brewer (71.2).