A study has been carried out to ascertain the effect of divalent cations and scale inhibitors on the lattice parameters, crystal size and morphology of barium sulphate. The mineral barite can support up to 6% calcium substitution for barium ions in the crystal lattice. Grahmann (Neues Jahrb. Min. 1 (1920) 1). It is known that the presence of magnesium and calcium ions in an inhibited scaling solution can greatly affect the performance of common scale inhibitors, and this study was designed to determine whether there were changes in the unit cell of the formed barium sulphate crystal. This paper reports that both the addition of magnesium ions and of very low levels of commercial scale inhibitor can greatly increase the amount of calcium incorporation. On correlating the calculated unit cell parameters with the amount of calcium in the precipitates, Vegard's law was found to hold, indicating solid solution formation. Magnesium in the reacting mixture increased the unit cell parameters of barite, and when magnesium and calcium were both present, increased levels of calcium were incorporated. The type and concentration of additive had a large effect on the size and morphology of the crystals. The ratio of seawater ions to formation water ions in the reaction mixture also exerted an effect. The presence of calcium in the initial solution led to dendritic morphologies. (C) 2002 Elsevier Science B.V. All rights reserved.