Fully relativistic ionization spectra of hydrogen iodide (HI) and the alkali iodides MI (M = Li, Na, K, Rb) are calculated using the one-particle propagator technique in the four-component framework. The algebraic diagrammatic construction (ADC) technique up to third order is hereby used for the matrix construction. Comparison to experimental results is made and the trends in the ionization spectra with increasing ionicity of the species investigated. The spin-orbit splitting in the outer valence and sub-valence ionization peaks is well reproduced in all cases. (c) 2005 Elsevier B.V. All rights reserved.