The photoelectron spectra of diborane, hexadeuterodiborane, borazine and B-trifluoroborazine are presented, and adiabatic and vertical ionization potentials have been measured. The vibrational fine structure observed on some of the diborane bands is shown to be consistent with the forms of the molecular orbitals calculated by rigorous s.c.f. methods. The vertical i.p. of diborane are in better accord with a calculation which predicts a boron-boron bond in addition to the hydrogen bridge than with the calculations which indicate no direct boron-boron interaction. In borazine it is shown that the uppermost orbital is of $\pi $ type rather than the $\sigma $ type predicted by calculations, and that the extent of the $\pi $ bonding, as measured by the energy separation of the $\pi $-type orbitals, is about 85% of that in benzene. The effect of fluorination of borazine, as in benzene, is to stabilize the $\sigma $ orbitals more than the $\pi $ orbitals.