The absorption of dyestuffs by montmorillonite can be attributed to two different processes, namely (1) base exchange and (2) van der Waals adsorption. There are some mixed cases, but usually one of the two effects is predominant and can be easily distinguished by several of the following features (see table 1.).

This paper deals with the base exchange type of adsorption only and in particular with the quantitative relations between the adsorbed dye and the released exchangeable cations. The simplest assumption, which is very often taken for granted, is that the two quantities are equivalent and correspond to the base exchange capacity of the montmorillonite.