Various amounts of interlayer Mg++, Ca++, Ba++, or Na+ of vermiculite were found to become "trapped" in the interior of the particles and rendered difficultly exchangeable upon replacement of the most accessible portion of these cations by NH4+ or K+. Collapse of the interlayer space of the crystal lattice adjacent to the edges of the particles, which results from the adsorption of NH4+ or K+, was advanced as the cause of "trapping." Variation in particle size of vermiculite does not affect adsorption of NH4+ or K+; adsorption of the latter ions is smaller, the larger the particles and the more perfect their degree of crystallinity. The cation-exchange capacity can be determined, therefore, more properly by measuring adsorption of the former than the latter cations. Adsorption of Ba++ is the most practical method, since the Ba++ - saturated vermiculite can be destroyed readily by digestion with 5 [image] HCl and the Ba++ detd. in the resulting soln. Exchangeable H+, when present, can be replaced only partly by a neutral salt soln. All of it, however, is replaceable by a basic salt soln. or by a boiling neutral soln. of BaAc2. The latter method was developed into a quantitative method for measuring exchangeable H+ One soil exhibited the property of "trapping" in a manner similar to that found in the vermiculites.