A test fluid of polyisobutylene,in a mixed solvent of polybutene oil and Decalin was prepared and sent to a number of laboratories around the world. This paper reports the results of experiments on this fluid in shear and extension. In steady shear flow, the data were fitted to a Carreau-Yasuda model equation at each temperature. Shift factors relating the data at any particular temperature to that at the reference temperature of 20-degrees-C were fitted to the Williams-Landel-Ferry (WLF) equation. Further data were obtained of values of eta' and G' with frequency. It was notable that the Cox-Merz rule did not apply to this fluid. The results of several different types of elongational experiments are given, and the methods are compared.