A test fluid consisting of polyisobutylene in a mixed solvent of polybutene oil and decalin was prepared, and was named S1. From data gained whilst preparing this test fluid, it was shown that, whilst the various constituent fluids are stable at rest, S1 showed an increase in viscosity with time when sheared at a constant stress. Unlike anti-thixotropy, this change was not reversible, at least during the usual time scale of experiments, and continued to occur even when the shearing was temporarily halted. The change is partly attributable to evaporation of Decalin. In solutions of polyisobutylene in Decalin, these changes in viscosity with time are present to a lesser degree. This paper also describes the physical and rheological properties of the constituent parts of S1. The polybutene oil was found to be a second order fluid, with a time constant of about 200 mus. 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. The mixed solvent was also tested. Solutions of the polyisobutylene in Decalin were tested in steady shear flow, and the data at each concentration were fitted to a Carreau-Yasuda model equation. Oscillatory and steady shear data were compared and the Cox-Merz rule found to hold.