Isopycnic focusing is described by a physical model based on equilibrium transport phenomena and derived from a general isoperichoric focusing theory. The model predicts the appearance of the focusing phenomena in bidisperse suspensions of nanosize colloidal particles of very low average particle size ratio approaching one. This prediction is in contradiction with the generally accepted supposition that a density gradient-forming liquid should behave as a continuum with respect to the focused species. Experimental verification of the theoretical approach was done by using a mixture of polyaniline composite colored particles and silica composite particles. Polyaniline nanosize particles exhibited the focusing phenomenon in a density gradient formed by colloidal silica particles under the effect of centrifugal field forces. Comparison of the experimentally found concentration distribution of the polyaniline particles in the focused zone with the theoretically calculated distributions confirmed the reliability of the theory if silica interparticle interactions are taken into account.