The paper compares the viscous, high-elastic and viscoelastic properties of solutions in various solvents of four polymers (polyisobutylene, polystyrene, polymethyl methacrylate and acetyl cellulose) differing in chain rigidity and the intensity of molecular interaction, over a wide range of compositions for the linear region of the mechanical behaviour of these systems. For solutions of polyisobutylene and polystyrene, the effect of the nature of the solvent on the viscosity is determined by the inherent viscosity of the solvent and by the fact that solutions under isothermal conditions are not in corresponding states with respect to their glass transition temperatures. The high-elasticity modulus of polyisobutylene and polystyrene solutions is independent of the nature of the solvent and of temperature. This indicates that the fluctuating network of entanglements is insensitive to the nature of the solvent and that the free volume plays the determining role in the behaviour of these solutions. Solutions of acetyl cellulose and polymethyl methacrylate are characterized by a very strong dependence of the viscosity on the nature of the solvent (which cannot be explained by notions valid for solutions of the first two polymers) and also by a change in the highelasticity modulus with variation in the nature of the solvent and temperature. This proves that the properties of solutions of polar polymers are determined both by the free volume and the density of the fluctuating network, which changes with the solvent and temperature. The shape of the relaxation spectra of solutions of various polydisperse polymers in solvents of different natures is the same, while the specific properties of the solutions are associated with the position of the spectra on the time scale, which determines the fundamental constants of the solutions, viz. the zero shear viscosity and the initial high-elasticity modulus. * To whom correspondence should be addressed. 85 * The molecular weights were measured by A. A. Anikeyeva, to whom we express our deep gratitude.