Multicomponent adsorption equilibria of organic components from aqueous solutions on activated carbons were measured and the applicability of the ideal adsorbed solution (IAS) theory was tested. Systematic deviations between experiments and theory were observed. Adsorbed phase activity coefficients which account for thesedeviations proved to be thermodynamically inconsistent. However, the calculation of spreading pressure in the IAS theory requires extrapolation of single solute isotherms to zero concentration. A correction of extrapolation errors was determined by fitting binary equilibrium data to the IAS theory. For a mixture of N components, N -1 constants have to be obtained from binary mixtures of N -1 components with a reference component. For the adsorption of 3 chemically non-interacting organic solutes constants derived from two binary systems allow the prediction of the third binary system as well as of the ternary system. The method is successful only if chemical interactions of the components in the adsorbed phase can be excluded. lNTRODUC!TlON Adsorption onto activated carbon has proven to be an effective technique for the treatment of industrial waste-waters and for the purification of the effluents from biological treatment plants. Industrial and municipal wastes usually contain a wide spectrum of substances which compete for available adsorption sites on the surface of the activated carbon. The design of equipment for the adsorption process requires knowledge of thermodynamic equilibrium in the system involved. For multicomponent systems experimental equilibrium data are difficult to obtain. Frequently the practical way of estimating multicomponent adsorption equilibria is to predict mixture isotherms from isotherm data of the single solutes. Severaral methods have been proposed (Markham and