Isotherms for adsorption of organic solutes are divided into four main classes, according to the nature of slope of the initial portion of the curve, and thereafter into sub-groups. The main classes are: (i) S Curves, indicative of vertical orientation of adsorbed molecules at the surface. (ii) L Curves, the normal or "Langmuir" isotherms, usually indicative of molecules adsorbed flat on the surface, or, sometimes, of vertically oriented adsorbed ions with particularly strong intermolecular attraction. (iii) H Curves ("high affinity") (commencing at a positive value on the "concentration in solid" axis), often given by solutes adsorbed as ionic micelles, and by high-affinity ions exchanging with low-affinity ions. (iv) C Curves ("constant partition"), linear curves, given by solutes which penetrate into the solid more readily than does the solvent. The sub-groups of these classes are arranged according to the shape of the parts of the curves farther from the origin, and the significance of plateaux and changes of slope are described. Thus, if the adsorbed solute molecules in the monolayer are so oriented that the new surface they present to the solution has low attraction for more solute molecules, the curve has a long plateau; if they are oriented so that the new surface has high attraction for more solute, the curve rises steadily and has no plateau. The choice of solutes for reliable measurement of specific surface areas is very restricted. It is suggested that p-nitrophenol may be one of the best compounds for this purpose.