The extensional viscosity characteristics of a terpolymer synthesized from acrylic acid, n-butyl acrylate and 2-ethyl-hexyl acrylate and used as a pressure sensitive adhesive, have been examined. By using a tensile test instrument it has been shown to be possible to generate three-dimensional plots of instantaneous extensional viscosity, Hencky strain and time of extension. The terpolymer was found to be strain hardening. During extension it also formed fibrils due to partial failure of the adhesive bond with an aluminium surface. The generation of the fibrils could be related to rate of extension and hence to the strain hardening behaviour.