A new chemically modified polymeric resin, with an o-carboxybenzoyl moiety, is developed to be used in the on-line solid-phase extraction (SPE) of some pesticides and phenolic compounds from aqueous samples, in order to obtain better breakthrough volumes than other commercial sorbents. The chemical introduction of this moiety improves the efficiency of SPE by providing better surface contact with aqueous samples. This synthesized sorbent enables higher volumes of sample to be concentrated for determining polar compounds. When analyzing real samples, tap and river waters, different quantities of sulphite were added to decrease the initial band due to fulvic and humic acids. The matrix peaks decreased and there were no losses in the studied compounds observed when 500 and 1000 microliters of 10% Na2SO3 solution for every 100 ml of sample were added to tap and river water, respectively.