The Saline Water Conversion Corporation (SWCC) of Saudi Arabia is an internationally recognized authority on scale control in multi-stage flash (MSF) distillers. This international recognition came about as a result of many years of work on this topic. There are publications of SWCC on MSF scaling phenomena over the past couple of decades. During the last decade, there was a monumental growth in SWCC＇s installed MSF plant capacities. The majority of SWCC MSF distillers were designed to operate with the additive scale control method rather than the previously predominant method of carbonate depletion through chemical reaction with sulfuric or hydrochloric acid. The Research and Development Center (RDC) in Al-Jubail has been involved during the last 3 years in the evaluation of available commercial scale inhibitors. Eight tests have been carried out at the RDC MSF pilot plant, Jeddah Phases III and TV and Al-Jubail Phase II to evaluate the effectiveness of three anti-sealants. These tests by anti-sealant types and places are (1) polycarboxylates in Al-Jubail II, RDC pilot plant and Jeddah IV; (2) polymaleic acid in Jeddah TV and RDC pilot plant; and (3) polyphosphonate in Jeddah III and the RDC pilot plant. This paper presents qualitative and quantitative performance results of three different anti-sealants. The threshold effect of these anti-sealants was tested on laboratory scale. In pilot and commercial plants the variation of the fouling factor with time was examined and quantified. The evaluation tests conducted on the commercial plants revealed that all the examined anti-scalants were successful in inhibiting alkaline scale formation and improving plant performance at a top brine temperature ranging between 105 and 110 degrees C and with respective dosing rates of 1.5 and 3.0 ppm.