The nonlinear behavior in simple shear of polyisobutylene solutions in cetane was studied for three concentrations in various shear histories. The concentrations of the solutions were 10, 15.1, and 19.3% by weight. Measurements of the shear stress and the first normal stress difference were obtained as a function of time at shear strain histories involving suddenly applied shear and stress relaxation after steady shear. Steady state values were also obtained for the shear stress and first normal stress difference as a function of shear rate, and the dynamic viscosity,η′(ω),and rigidity,G′(ω)were obtained as a function of frequency. Some isochronal data is also presented from single step stress relaxation experiments. It is shown that a simple superposition principle can be applied at concentrations where the intermolecular forces are predominant for the systems and for which the potential function in the Bernstein, Kearsley , and Zapas elastic fluid theory can be expressed as a product of a function of time and a function of strain. An excellent agreement was obtained with the experimental data, even for transient experiments where the shearing and normal stresses depend on time.