The effect of calcium cations on the precipitation of barium sulphate is comprehensively studied using several techniques for the first time. The dominant mechanism at <0.00025 M calcium ions is an ionic strength effect. Above this concentration, tests conducted at the same ionic strength show that differences in the behaviour of barium sulphate precipitation occur depending on the cation present (sodium or calcium). This difference is believed to be due to one or both of the following mechanisms: calcium incorporating into the lattice and increasing the internal energy of the crystal and/or calcium adsorbing on the surface and poisoning growth. In the absence of organic additives, the presence of calcium was not observed to promote either nucleation or precipitation in general. (C) 2003 Elsevier B.V. All rights reserved.