Anhydrous potassium benzene sulphonate shows unusual powers of sorbing both non-polar and polar molecular species, often in large quantities. Isotherms have been obtained for various paraffinic, cyclo-paraffinic and aromatic hydrocarbons, for some heterocyclic molecules, for alcohols from methanol to normal and tertiary butyl alcohol, and for other sorbates including carbon tetrachloride, n-perfluoroheptane, ethylamine and water. Many of these isotherms reveal hysteresis associated with nucleation in and subsequent recrystallization of the host crystals of sorbent. The kinetics of sorption can also in many of the sorptions be interpreted quantitatively as growth from nuclei appearing in numbers proportional to the square of the time. In other cases the kinetics follow a √t diffusion law. Potassium benzene sulphonate functions as a sorbent in several ways. Sorption may be confined to external surfaces (cyclohexane, n-perfluoroheptane and iso-octane). With other sorbates lattice penetration and structural rearrangements occur. Structures similar to the so-called ‘quarter-hydrate’ may be formed between the sulphonate and some aromatic hydrocarbons, while structures similar to the ‘hemi-hydrate’ may be obtained with some polar molecules (pyridine, thiophene, dioxane, ethylamine). At lower relative pressures of sorbate vapour intermediate stages of intracrystalline rearrangement, preceding formation of quarter- and hemi-hydrate structures, can be detected by X-ray study. Such intermediate stages represent also the limit of sorption by n-heptane, cyclopentane, carbon tetrachloride and t-butanol. With several sorbates the degree of swelling and rearrangement appears to go beyond even the stage of the hemi-hydrate structure. The quarter- and hemi-hydrate structures appear to be zeolitic in properties. Thus the proportions of sorbate to sorbent are not stoicheiometric, and apparent saturation values often decrease with rising temperature. If sorbate is removed by suitably lowering the pressure of the ambient sorbate vapour the zeolitic lattices collapse and revert into intermediate structures less rich in sorbate and finally into sorbate-free sulphonate crystals. Steric factors of shape and size in the sorbate molecules help to determine how much of each species is occluded, and what structures are eventually formed. The polarity of the sorbate molecules is a second important factor governing the sorption process. The crystals have been used in several instances to separate the components of liquid mixtures.