The multicomponent approach in the description of molecular diffusion takes into account the correlation of motion between solutes. Here it is shown that this approach is necessary to describe correctly the flows of each component in concentrated solutions of macromolecules, defined "crowded solutions". Microscopic intuitive considerations make clear this necessity also for hypothetical uncharged hard particles. We present precise measurements of mutual diffusion coefficients relative to five compositions of the ternary system poly(ethylene glycol) (PEG) 400-NaCl-water, changing both the PEG and salt concentrations. The values of the experimental diffusion coefficients for this system seem to be dominated by an excluded volume effect. Our recent predictive equations, proposed to evaluate the diffusion coefficients in a ternary systems of hard sphere solutes, have been tested on the experimental data with reasonable success. The thermodynamic and gravitational stability analysis on the collected data is also reported. By changing the choice of solvent constituent from water to PEG or NaCl, it is clear that the counter-flow related to the polymer flux in crowded solutions is due essentially to the water and not to other constituents.