Abstract The valence shell threshold photoelectron spectra and the photoionisation yield curves of the boron trihalides have been recorded using synchrotron radiation. The threshold photoelectron spectra demonstrate that spin–orbit coupling is important in the heavier trihalides and affects the bands associated with degenerate orbitals. In addition to the photoelectron bands associated with direct ionisation, features have been observed due to autoionising Rydberg and valence excited states. An analysis of the vibrational progressions occurring in the photoelectron bands has enabled an almost complete set of ionic vibrational energies to be determined. The continuous nature of the inner valence shell photoelectron bands and the absence of main-lines illustrate the importance of electron correlation in redistributing the intensity amongst numerous final states. In all four of the boron trihalides, new Rydberg series have been observed in the photoionisation yield curves. However, prominent broad features attributable to intervalence transitions dominate the photoion spectra. In some Rydberg series the intensity distribution amongst the members appears irregular, and this may be due to mixing between Rydberg and valence excited states. Several, rather weak, vibrational progressions involving the ν 1 ′, ν 2 ′ and ν 4 ′ modes have been observed.