The adsorption of herbicides alachlor, amitrole, trifluralin and prometryn on porous polymeric adsorbents has been studied. Two adsorbent resins were investigated, the highly hydrophobic Amberlite XAD-4 (polystyrene-divinylbenzene copolymer) and the functionalized more hydrophilic XAD-7 (nomonic aliphatic acrylic polymer). Equilibrium adsorption experiments using buffered aqueous solutions were conducted to estimate the types of isotherms and their parameters. The effect of chemical composition and structure of the herbicides, was investigated. The pH range studied was 3-6.5, the temperature range was 288-303 K and the ionic strength was maintained at 0.01 M. Adsorption isotherms seemed generally to approach the Langmuir or Freundlich isotherm model and can be characterized by temperature and pH dependent apparent adsorption equilibrium constants, characteristic of the adsorbent-adsorbate system. By studying the dependence of temperature of this adsorption constant, heats of adsorption have been estimated from van't Hoff law. In the case of trifluralin and prometryn adsorption on both resins and amitrole adsorption on XAD-4 resin, the heats of adsorption were negative (8.1-33.6 kcal/mol). On the contrary, in alachlor adsorption on both resins and amitrole adsorption on XAD-7 resin, the estimated heats of adsorption were positive. (C) 2004 Elsevier Ltd. All rights reserved.