A linear isotherm model, modified to account for a non-adsorbable fraction, was found to fit adequately the adsorption equilibrium of organic matter from a tertiary treated wastewater. The isotherm slope varied significantly among sample days, but the range of variability did not exceed a factor of two. A moderate ozone dose (0.5 g O3/g DOC) enhanced adsorbability in both laboratory and full-scale systems, whereas higher doses appeared to decrease adsorption. Preozonation had little effect on the nonadsorbable portion of DOC.