Additional data are presented for a bubble inflation technique which was used to develop equal biaxial extensional flow in viscoelastic materials. Biaxial extensional viscosity and elastic properties of a polyisobutylene at 23°C are compared with the predictions of constitutive equations and with experimental results obtained from other instruments. Constant pressure (rather than constant tension) experiments were carried out to the point of bubble bursting. A modified plot involving variables derived from pressure and bursting time is compared with the biaxial viscosity data, and the prospects for using this simpler test are discussed.