A bubble inflation technique for establishing equal biaxial extensional flow in viscoelastic materials is presented. This technique was used to measure biaxial extensional viscosity and elastic properties of a polyisobutylene at room temperature (23°C). A theoretical experimental procedure, based on certain idealizations, was developed for establishing bubble growth under constant stress. Modifications of the experimental design were introduced to correct for the nonidealities encountered in practice. The accuracy and reliability of the measurements are tested, and sources of error, and possibilities for future work are discussed. Several constitutive equations are examined with respect to biaxial extensional flow. Predicted behavior patterns are compared with experimental results.