The process of diffusion of sodium hyaluronate and its hydrophobically modified alkyl derivatives in water was studied by measuring the kinetics of solid polymer film swelling. The following fundamental thermodynamic parameters of the swelling process were calculated: the apparent diffusion coefficient of swelling by water D(s) determined at three temperatures (25, 37, and 45 degrees C), the activation enthalpy of diffusion connected with the swelling DeltaH(D,s) and the activation enthalpy of the swelling process DeltaH(s). The thermodynamic activity of the solvent for the given sodium hyaluronate and its four alkyl derivatives has been expressed through the internal quantity RA(delta,s), that is expansion work of polymer coil accomplished by the action of the internal pressure.