A highly nonequilibrium system of two miscible liquids is proposed as a generator of barium sulfate nanoparticles. A dynamic transition layer model developed in this work is used to explain the obtained results; according to this model, the smallest diameter of the resulting particles is determined by the size of the prephase associates formed in rotating Benard cells. It is hypothesized that the Benard cell-ambient solution interface has certain barrier properties, at least with respect to the new phase precursor’s particles penetrating from outside. The effect of the H2O/tetrahydrofuran ratio on the size of particles and the shape of associates and nanocrystallites is studied. The dependence of the size of nanocrystalline particles on the above ratio is derived. A possible explanation is proposed for the dependence of the size of nanoparticles on the viscosity of a solvent.