A combined X-ray photoelectron and Auger electron spectroscopic investigation of sorption and characterization of bonding states of Cs in montmorillonite (M) and in four reduced-charge montmorillonites (RCMs) were carried out. The nondestructive analysis of sorption of Cs in variable-charge montmorillonites obtained by XPS is consistent within 4-8% to those obtained by solution analysis using AAS. The core-level Cs 3d5/2 photoelectron binding energy was observed to be enhanced with the reduction of electron population from higher to lower charge montmorillonite. The charge potential model has been used to interpret such core-level short-range chemical shifts. However, a larger shift in the Auger Cs-M4N4,5N4,5 line was observed and Cs-Auger kinetic energy was found to decrease from higher charge RCM to lower charge RCM, which may be attributed to the effect of extra-atomic relaxation from surrounding oxygen atoms; this effect implies that the flow of electronic charge to the Cs ion is larger for higher charge RCM than for lower charge RCM. However, this effect is enhanced by the extra contribution of Li that has migrated. A comparison of Cs 3d binding energies and Cs-Auger kinetic energies in a two-dimensional chemical state plot indicates multiple bonding states of Cs in variable-charge montmorillonites. The extra-atomic relaxation energy (DeltaEr) value of each of the Cs-montmorillonites was unique, which indicates that this is dependent on the chemical environment and on the availability of electron density in montmorillonites. Also, the binding energy value of Cs 3d5/2 was found to be nearer to CsOH than to CsCl, which reflects that the bonding state of Cs is not as ionic as CsCl. Copyright 1999 Academic Press.