For a commercial polyisobutylene, at room temperature, stress relaxation after a sudden imposition of a shear or elongational strain has been performed in the limit of linear viscoelasticity. Transient shear and elongational stresses have been determined in the non‐linear region by using a Universal Instron Testing Machine. While for shear flow steady‐state conditions were reached in some cases, the elongational results are only transient because the experimental procedure gives a rate of stretching which decreases in time. Some steady shear results have been obtained at higher temperatures with a capillary viscometer. The results have been interpreted by means of a one‐parameter model which attributes the non‐linearities arising from the change in the spectrum of relaxation times to modification of the entangled network. The Bogue‐White model has been also considered for comparison.