A new calculation method for multicomponent adsorption equilibria was proposed based on the real ad-sorbed solution theory(RAST) and the concept of arbitrary isotherm. According to the latter, we assigned a respective isotherm, which gave the best fit to single-component equlibrium data, to each component, instead of using a specific type of isotherm for all adsorbates as in most previous works. This enabled us to evaluate the spreading pressure for pure components more accurately, which will be used in multicomponent adsorption calculations later. The usefulness of the proposed method is verified by carrying out sample calculations for ternary data reported in literature.