A high-pressure sliding plate rheometer was used to investigate the flow behavior of polyisobutylene in simple shear. Experiments carried out using smooth steel surfaces revealed that wall slip is a dominant feature of the flow at the pressures, temperatures, and rates of deformation typically found in the processing of elastomers. A set of grooved plates made it possible to obtain viscosity data at stresses up to 20 kPa, but at higher stresses, the sample "slipped" even on the grooved plates. These data were fitted to a Cross viscosity model and extrapolated in order to estimate the viscosity at higher stresses. Flow curves (shear stress versus nominal shear rate) generated using fluoroelastomer-steel and steel-steel pairs of plates exhibited four distinct flow regimes: no-slip, adhesive slip, mixed adhesive, and cohesive slip, and primarily cohesive slip. Slip velocities were calculated by comparing the no-slip stresses estimated using the Cross model with measured values. The effect of pressure on cohesive wall slip was found to scale with pressure in the same way as viscosity, while the effect of pressure on adhesive wall slip did not.