The formation potential of haloacetonitriles (HANsFP) from chlorination of raw water of the Dez River in Iran was determined. Samples were collected before treatment at an intake of a water treatment plant. Tests were carried out to determine the effect of the reaction time (5-120 h), water pH (4-10), total organic carbon (TOC) content (2-8 mg L(-1)), chlorine dosage (3-25 mg L(-l)), water temperature (5-30 degrees C) and usual coagulants on HANsFP in the Dez River water. Increasing chlorine reaction time up to 80 h led to the remarkable increase in HANsFP and after that the HANsFP was gradually increased. Over 90% of the HANs was formed during 80 h reaction time. Linear relationships were found between the HANsFP and the chlorine dosage. The result showed that the HANsFP increased by increasing TOC content up to 6 mg L(-1), while further increases in the TOC level did not lead to any significant changes in the HANsFP level. The acidic water pH resulted in the highest HANsFP. An increase in water temperature enhanced the HANsFP; this factor should be considered for tropical waterworks. The ferric chloride had a high efficiency for the removal of HANsFP (72%). Overall, the Dez River water had a high potential for forming the HANs (approximately 17 microg L(-1)).