The primary objective of the study described in the present paper was to investigate the problem of how the concentration of organic matter (expressed as dissolved organic carbon, DOC) contributes to the efficiency of separating atrazine form aqueous solutions by ultrafiltration. Another objective was to examine how the type of organic matter (humic substances HS, tannic acid TA) may affect the efficiency of atrazine separation. A series of experiments was carried out with model solutions containing atrazine (1000 mg/m(3)), humic substances (10, 20, 50 g/m(3)) or tannic acid (5, 10, 15, 20 g/m(3)) at a neutral pH. The ultrafiltration process involved a DS-GE polysulfone/polyamide membrane (Osmonics) and a transmembrane pressure of 0.1 and 0.3 MPa. The highest 3 atrazine rejection coefficient (80%) was achieved at the organic matter concentration, DOC = 8.1 g C/m(3).