In supersaturated solutions the period preceding the start of 'measurable' crystallization is normally referred to as the 'induction time'. This research project aimed to investigate the induction times of CaCO3 in the presence of Mg2+ and SO42-. The prepared synthetic solutions have the same ionic strength values found in the Gulf of Oman SWRO concentrates at 30% and 50% recovery. The results showed a significant increase in the induction time by 1140%, 2820%, and 3880% for a recovery of 50%, when adding SO42- only, Mg2+ only, or both Mg2+ and SO42- respectively, to synthetic SWRO concentrate compared to that obtained in the absence of Mg2+ and SO42- at an initial pH of 8.3. The increase in the induction time in the presence of SO42- was more than likely to be due to nucleation and growth inhibition while the presence of Mg2+ affected the nucleation and growth through both complexation and inhibition. After a 5-month solution stabilization period, ESEM and XRD analyses showed aragonite in solutions containing Mg2+. On the contrary, calcite was the final crystal phase formed in solutions with no Mg2+. This suggests that magnesium may play an important role in inhibiting the formation of calcite. (C) 2011 Elsevier B.V. All rights reserved.