Precipitation of barium sulphate from aqueous solutions was investigated in a tubular reactor. Crystal size distribution and morphology were measured at the reactor outlet. Experimental data were compared with model predictions using CFD coupled with a micromixing model. Some anti-symmetric effects of ion excess on precipitation were observed. Activity coefficients were used in modelling precipitation, but even if thermodynamic properties of the solutions plays a role in determining nucleation and growth rates, this approach alone is not sufficient to give satisfactory agreement.