Production of fine particles by precipitation is a relatively simple process but the control of product particle size distribution can be difficult. In recent years microreactors are being seen as effective devices to achieve this aim and in that the two-phase flow microreactor is a promising alternative. In the present work, fine (nano/micro) particles of barium sulfate have been produced by reacting barium chloride and sodium sulfate solutions in a 30 cm long and 830 mu m inner diameter glass capillary microreactor. The T-inlet section has all three limbs 15 mm long and 1 mm in diameter. Particle sizes produced ranged from about 200 nm to 1100 nm, with mean sizes between 300 nm and 670 nm. Results show that an increase in the flowrate of any of the three streams, the two reactant liquid streams and one inert gas stream, led to a decrease in the mean particle size of the precipitated barium sulfate product. Increased mixing and higher Ba2+ ion concentrations in the small microreactor (liquid slugs) are believed to shift rates in favour of nucleation as compared to agglomeration, yielding smaller particles in the product. Size variation results show a trend similar to that reported in earlier researches on BaSO4 precipitation in single (liquid) phase microreactors but the actual sizes obtained in the present work are bigger. Other geometries and flow conditions may yield smaller particles, hence the need for further investigation of the two-phase flow system. (C) 2011 Elsevier B.V. All rights reserved.