The absorption of dyestuffs by montmorillonite can be attributed to twodifferent processes, namely (1) base exchange and (2) van der Waalsadsorption. There are some mixed cases, but usually one of the two effectsis predominant and can be easily distinguished by several of the followingfeatures (see table 1.).

This paper deals with the base exchange type of adsorption only and inparticular with the quantitative relations between the adsorbed dye and thereleased exchangeable cations. The simplest assumption, which is very oftentaken for granted, is that the two quantities are equivalent and correspondto the base exchange capacity of the montmorillonite.