Absorption spectra of solutions in acetonitrile of tetra (ethyl) ammonium salts of UCl6--, UBr6--, UI6--, NpCl6--, NpBr6--, PuCl6--and PuBr6--show three types of bands arranged according to increasing half-width: (1) internal 5fqtransitions showing a large nephelauxetic effect, the wavenumbers in the hexabromides being some 3–5 per cent smaller than of the corresponding hexachlorides; (2) 5fq→5fq-16d transitions indicating that the 5f→6d orbital energy differences are roughly constant as function ofqin contrast to the strong increase in the aqua ions; (3)π245fq→π235fq+1electron transfer bands indicating that the optical electronegativitiesxopt(corrected for spin-pairing energy and certain effects ofL, S, Jsplitting) are 1·5 for U(IV), 1·75 for Np(IV) and 2·05 for Pu(IV). Further on, the charge separation effects on electron transfer bands are discussed with special regard to uranyl iodide complexes.