Ultraviolet photoelectron spectra of UBr4 and ThBr4 have been recorded in the gas-phase and interpreted using relativistic density functional calculations. For ThBr4, eight bands were observed which are interpreted as ionization from the five Br 4p symmetry orbitals of a Br-4 unit in T-d symmetry, with three of the bands [the (4t(2))(-1), (1t(1))(-1), and (3t(2))(-1) ionizations] each being split into two by spin-orbit interaction. The observed splittings are rationalized in terms of the Th 6p and Br 4p contributions to the orbitals and by comparison with the known ultraviolet photoelectron spectra of ThF4 and ThCl4. The first vertical ionization energy (VIE) of ThBr4 was measured as (10.92 +/-0.03) eV. UBr4 shows a very similar photoelectron spectrum with an extra band at (9.65 +/-0.02) eV VIE. This is associated with a (5t(2))(-1) (U 5f,6d) ionization. Supporting matrix isolation infrared experiments were also carried out under very similar vaporization conditions to those used in the photoelectron spectroscopy experiments to check the composition of the vapor beams used. In these experiments, the T-2 stretching modes of ThBr4 and UBr4 have been measured as 230 +/-2 and 239 +/-2 cm(-1), respectively. Both the photoelectron and infrared matrix isolation spectra are consistent with an effective tetrahedral geometry for UBr4 and ThBr4.