The competitive adsorption of aqueous humic substance onto activated carbon was characterized. A technique to determine its composition in terms of both the molecular weight and the adsorbability was developed. This technique required the equilibria of molecular weight fractions in the mixture and that of the hypothetical components which could account for the competitive adsorption of the total organics. The former was obtained by measuring the molecular weight distributions of the humic substance remaining in solution over various carbon dosages with size exclusion HPLC. The latter was achieved by using a method developed on the basis of ideal adsorbed solution theory (IAST) to describe the overall adsorption isotherms for background organics of known composition. Using this technique, the adsorption equilibria of all the molecular weight fractions in the mixture were quantitatively described, and the existence of a correlation between the molecular weight and the adsorptive strength was confirmed. This technique was shown to be also effective in clarifying the composition of the remaining humic substance over different carbon dosages.