Light scattering and viscosity measurements have been conducted on polystyrene samples (guest polymer) in pure toluene and in the isorefractive mixture toluene-poly(methyl methacrylate). The second polymer (host polymer) is present at constant concentration in the solution. In the dilute solution region in which the measurements are conducted, only when the solution flows does the host polymer provoke a decrease in the dimensions of the guest polymer, while in the static state the dimensions of the guest polymer are not affected by the presence of the host polymer.