Scale control additives (SCA) or antiscalants are used in multistage flash desalination plant to prevent fouling problems. Effective dose rates of some of these SCA are found to vary from plant to plant. Efficiency of antiscalants could be influenced by its chemical nature, plant configuration and design, heat transfer rate and its surface and antiscalant reaction with brine constituents. To understand the variation in concentration of SCA and effect of brine constituents on efficiency of SCA, some bench top experiments at 95 degrees C withbrine were conducted using the threshold effect technique. Two SCA, a polyphosphonate based (PPN) and polymaleic acid based (PMA) were used for these evaluations. The effect Fe3+, Fe2+, Fe2O2. Fe(OH)(3), and rust on efficiency of SCA was determined. The threshold effect method, which measures the total alkalinity and calcium content ofsupernant to evaluate efficiency of SCA, was found to be an effective technique. Recycled brine constituents and corrosion products, especially Fe and its various chemical forms affect the efficiency of SCA at 2 ppm concentration and at 95C in 1.4 times concentrated seawater brine. Various chemical hydrated andhydroxylated forms of Fe present in brine, viz., Fe3+(H2O)(6), Fe(OH)3, Fe2+, Fe2O3, rust, etc., all showed detrimental effect on efficiency of SCA. Among all the speciesof iron, Fe(OH)3, reduced the efficiency of SCA by maximum of 40%. Among the two antiscalants, PPN showed 10-12% better efficiency in the presence of various iron species compared to PMA under similar bench top experimental conditions. The effect of brine constituents on PMA was more pronounced compared to PPN perhaps due to better complexing ability of the former.