Using dynamic (viscometry) and static (light scattering) measurements we study solutions of polystyrene and poly(methyl methacrylate) fractions ranging from extremely dilute solutions up to the critical concentration c*. When viscometry is used, a crossover phenomenon is observed, separating the dilute solutions into extremely dilute and dilute solutions. The critical concentration c**, determined from this crossover phenomenon, depends on the hydrodynamic volume of the macromolecular coils and is lower when the molecular weight of the coils becomes higher, as has been observed for the critical concentration c*. By changing the shear rate, c** also changes and tends to disappear at a practically zero shear rate. The variation of c** with the shear rate implies that this crossover phenomenon is a dynamic phenomenon and for this reason it is not observed when using static measurements.