Tracer diffusion of 3000 g/mol perdeuterated polybutadiene (dPB) into high molecular weight polystyrene (PS) was measured with forward recoil spectroscopy (FRES) in the temperature range from 90 to 110-degrees-C in the presence of argon and helium pressures up to 11.3 MPa. The diffusion coefficient for dPB in PS at 107-degrees-C increases from 1.2 X 10(-13) to 8.7 x 10(-13) cm2/s as the argon pressure is increased from atmospheric pressure to 11.3 MPa. When helium is the pressurizing medium, the diffusion coefficient for dPB in PS at 107-degrees-C decreases from 1.2 x 10(-13) to 3.7 x 10(-14) cm2/s as the pressure is increased from atmospheric pressure to 11.3 Wa. The results are explained in the framework of competing hydrostatic pressure and plasticization effects. The solubility of dPB in PS is independent of pressure, irrespective of the gas used as the pressurizing medium.