Probe diffusion of camphorquinone (CQ) in a polystyrene (PS)/poly(2,6-dimethyl-1,4-phenyleneoxide) (PPO) blend is studied by using the laser-induced holographic grating relaxation technique. The probe diffusion coefficient is measured as a function of temperature and composition. Above the glass transition temperature, the temperature dependence of the diffusion coefficient fits well to the Vogel-Fulcher equation. At a fixed temperature above T-g, it is found that the logarithm of the probe diffusion coefficient varies linearly with the weight fraction of PS or PPO.