The Saline Water Conversion Corporation (SWCC) has been actively involved in the investigation of scale formation in order to improve the performance of its distillers. A number of optimization tests have been carried out resulting in successful operation of MSF distillers at low antiscalant dose rates. The additives which were tested include polymaleic, polyphosphonate and polycarboxylates. The present study reports the results of the tests which have been carried out to evaluate the effectiveness of polyphosphonate in controlling scale formation on the heat transfer surfaces at a top brine temperature (TBT) of 90.6 degrees C and a dose rate of 0.8 ppm, which is considered low compared to previous practices. These tests were firstly performed on a 20 t/d MSF pilot plant. The test results at the pilot plant showed stable overall heat transfer coefficients and fouling factors through out the test period, thus, indicated insignificant scaling of the heat transfer surfaces. Based on satisfactory results at the pilot plant, the test was performed in Al-Jubail Phase II at a TBT of 90.6 degrees C and dose rate of 0.8 ppm for a total test duration of 93 days. The overall heat transfer coefficient of the brine heater was almost constant at a value of 2700 W/m(2) K, thus consistently greater than the design value (2258 W/m(2) K). The unit performance ratio and distillate production rate were also fairly close to the design value. Chemical analysis results showed that product water conductivity and loss of total alkalinity of brine blowdown were maintained within acceptable limits. The brine heater and flash chambers were inspected at the end of the test. The brine heater tubes were found clean and also no sign of fresh scaling was noted on the demister pads, which confirmed the capability of polyphosphonate in controlling alkaline scale formation at TBT of 90.6 degrees C and a dose rate of as low as 0.8 ppm.