The effectiveness of granular activated carbon (GAC) adsorption for the removal of natural organic matter and trihalomethanes from Ivedik Water Treatment Plant of Ankara City is investigated. Freundlich Isotherm constants K and n were determined as 17.61 and 1.66, respectively. Bench-scale GAC columns were run with empty bed contact times (EBCT) varying from 0.40 to 2.67 min to evaluate adsorption performance. 50% exhaustion values were used for comparison. The treated volumes of water increased with EBCT, showing a linear increase in GAC service life. Correspondingly, the carbon usage rate decreased. The capacities calculated by the isotherm equation and achieved by columns were also compared. The column capacities were within 43-65% of the isotherm capacities at complete breakthrough. However, they were only within 8-17% of the isotherm capacities at 50% breakthrough. (C) 2002 Elsevier Science Ltd. All rights reserved.