A homogeneous liquid phase reaction between barium chloride (BaCl(2)) and sodium sulphate (Na(2)SO(4)) was conducted in a narrow channel reactor to produce barium sulphate (BaSO(4)) precipitate. The effects of channel dimensions and channel residence times on crystal size, crystal size distribution, nucleation rates, crystal morphology and conversion of reactants were investigated at different levels of reactant supersaturation ratio. Our results indicate that the smallest particle sizes are favoured when supersaturation ratios and channel velocities are high. The minimum average particle diameter observed was approximately 0.2 microm in a channel of hydraulic diameter 0.5 mm and length 20 cm at an initial supersaturation ratio of 4483 (0.1 M), which correspond to conditions giving rise to the highest nucleation rates. It has also been observed that particle size depends on the conversion to product, the smallest particles being formed when conversion lies within the range of 30 to 40%. Conversions in excess of 60% have been reached but there is a detectable limiting effect with increased supersaturation and reduced residence times. Experiments conducted at similar levels of supersaturation under stirred tank conditions showed that particle size was consistently larger and particle size distribution was much broader than that achieved in the narrow channel reactor. Scanning electron microscopy (SEM) images of the crystals formed in the narrow channels show that spherical particles dominate in the smallest channels at high velocities whilst coarse, tabular crystals are obtained in the larger channels. Greater tendency to agglomerate is also observed at high supersaturation ratios, after one minute of reaction.