The crystal habit of barium sulfate formed by the rapid mixing of Ba2+ and SO2–4 containing brines at 95 °C has been investigated and the effects of coprecipitated K+, Mg2+, Ca2+ and Sr2+ ions, supersaturation ratio and barium-to-sulfate mixing ratio on the crystal morphology studied. The most marked crystal morphological changes were induced by varying the supersaturation and mixing ratios. The foreign ions produced relatively subtle effects. At high supersaturations (within the homogeneous nucleation regime), eight-pointed star-like crystals were formed, whilst at low supersaturations, the equilibrium rhombohedral crystal form was recovered. All precipitates were single crystals. In the presence of crystal growth modifiers, marked morphological changes could be induced. For example, certain polymers induced the formation of millimetre long bundles of needles at pH ≈ 6 and fractal-like hollow cones at pH ≈ 5. In contrast, a range of phosphonate-based molecules, designed to act as barium sulfate scale inhibitors, produced oblate spheroids and very distorted star-like crystals 15–20 times smaller than the unmodified crystals. These particles were found to be porous on the nanometer scale. All the precipitates described were single crystals. We have found that the control and prevention of barium sulfate precipitation by phosphonate-based materials at high Ba2+ concentration and low pH (ca. 4.5) are complicated. In the absence of Ca2+ ions in solution, little or no inhibition occurs and evidence suggests that calcium phosphonate complexes are the active inhibitors. In addition, although the inhibitors may act by the classical mechanism of blocking crystal growth sites, they also act as nucleation promoters. This provides an additional mechanism for scale inhibition involving depletion of scaling ions from solution. As part of this work, using dynamic light scattering, we have for the first time demonstrated the presence of 1–10 nm microcrystallites in fully and partially inhibited barium sulfate scaling systems.