Barium sulphate scaling in reverse osmosis (RO) causes flux decline and potentially severe membrane damage. Du Pont＇s method to predict BaSO4 scale based on predicting barium solubility in RO concentrates is unreliable and limited to 25 degrees C. This method predicted barium solubility was exceeded 14 times at 80% recovery and yet no scaling occurred at the pilot plant. Possible explanations are; inaccurate solubility prediction, low rate of BaSO4 precipitation and/or organic matter effects on solubility or precipitation. This study investigated barium solubility in RO and the effect of, e.g., ionic strength using more theoretical approaches to solubility prediction i.e. Bromley and Pitzer models. Seeded growth determination of barium and synthetic concentrates (no organic matter) confirmed supersaturation and proved organics had no effect on solubility. Du Pont＇s method under predicted solubility by ca. 30%. The Pitzer and Bromley methods when calibrated for RO concentrates by applying an experimental K-sp gave accurate prediction at 5-25 degrees C for the ionic strength range of 0.01-0.1 M. For higher ionic strengths, the Pitzer model was more accurate. The observed stable supersaturation (27 times the solubility at 5 degrees C) in the pilot plant is most likely due to the low rate of precipitation and is under investigation.