The most important step in a reverse osmosis (RO) plant is the pretreatment of feedwater, One of the most common pretreatment methods used to prevent or control the scaling of different salts in seawater feed is the dosing of scale inhibitor. The desalination market yearly creates new types of inhibitors that claim to increase recovery and ensure stable operation without scale problems. RO plant operators must evaluate these chemicals before and after use in real plants. Therefore, the scale inhibitor must be evaluated on laboratory bench-scale before being used in the real plant and optimization must be done to choose the correct level of dosing to be applied in the plant. During operation, detailed investigation of different operation parameters has to be carried out to evaluate the antiscalant performance. The Doha research plant (DRP) in Kuwait with a RO unit (capacity of 300 m(3)/d) has been operated with two different types of antiscalants. This paper discusses the bench-scale evaluation results and different operation parameters for each antiscalant, i.e., the chemical analysis of the brine, the product conductivity, differential pressure across membrane, product flow, salt rejection, and number of cleanings required. A cost comparison is also made between the two inhibitors. It is recommended to continue operation with the present antiscalant due to its high performance and low cost.