The precipitation of barium sulfate from aqueous solutions of barium chloride and sodium sulfate was investigated experimentally in a continuous Y-Mixer to avoid any mixing influence. Initial supersaturation and the initial ratio R = c(Bafree)2+ /c(SO4.free)2- of the free lattice ions after complete educt mixing were varied over a broad range. The variation of the free lattice ions has a strong influence on the obtained particle size distribution. At low supersaturations precipitation experiments with barium or sulfate ion excess lead to smaller mean particle diameters compared to experiments done under stoichiometric conditions. At higher supersaturations a sulfate ion excess has no effect on the detectable particle size. Streaming potential measurements reveal that the particle charge increases only with increasing barium ion excess. Higher repulsive electrostatic forces are caused by specific adsorption of the lattice ions on the particle surface and aggregation of primary particles can be avoided. By variation of supersaturation a broad range of particle sizes can selectively be produced, ranging from micrometer sizes down to nanoparticles smaller than 50 nm. The variation of supersaturation and of free lattice ion ratio at constant supersaturation has a great influence on crystal morphology. (c) 2006 Elsevier B.V. All rights reserved.