Abstract: Fifteen pure montmorillonites selected on the basis of minor differences in x-ray diffraction and differential thermal analysis data were treated with ethylene glycol, piperidine, and dodecylamine. X-ray and differential thermal data of the resulting complexes lead to the following conclusions: The complexes of montmorillonites with a given organic compound show differences ascribable to differences in the character of the montmorillonites. Attempts to relate variations in organic complexes to exchangeable cations, chemical composition, and collodial properties of the montmorillonites met with very limited success. All montmorillonites are mixed-layer sequences; i.e. adjacent layers differ from one another in composition, structure, or some other factor. It is suggested that mixed layering of this nature may also occur in other three-layer clay minerals.