Results are presented of a detailed rheological study of two different polyisobutylene systems in steady and transient shear flows. Shear‐thinning solutions are prepared by dissolving high molecular polyisobutylene in tetradecane, a low viscosity solvent. Nearly constant viscosity solutions are obtained by adding a third component of highly viscous polybutene. The linear viscoelastic behavior of these fluids is characterized in small‐amplitude oscillatory shearing flow and the nonlinear behavior is studied by means of steady and transient shear flows. The spectrum of relaxation times for both solutions has been determined. Temperature and concentration effects on the material functions are reported. Three differential constitutive equations with four relaxation modes were tested for fitting the experimental data in steady and transient shear flows: they are the quasilinear Oldroyd‐B model and the nonlinear Giesekus and Bird–DeAguiar models. The presence of nonlinear stress terms is found to be important for fitting the nonlinear material functions for both types of viscoelastic solutions.