The ionization energy I of the fluoride Is shell is shown to vary from 695.7 to 687.8 eV in 40 solid compounds and has been compared with the value for four gaseous fluorides from the literature. The influence of the Madelung potential on potassium compounds and the combined influence of changing the oxidation state and the Madelung potential on iodine compounds have been studied from a comparative viewpoint. Central atoms with high oxidation numbers and small radii produce the highest I values in fluorides. The far-UV spectra and polarization effects in solid halides have also been discussed.