Diffusion of the radioisotope Rb-86 in an amorphous polymer-salt complex consisting of poly(ethylene oxide) and sodium iodide was found to be faster at all temperatures investigated than tracer self-diffusion of the smaller alkali metal cation Na-22. This is the striking result of the first study on impurity diffusion in a polymer electrolyte system and a comparison with ionic self-diffusion and conductivity data previously obtained from the same system. The experimental findings can be rationalized within an ion transport model based on the occurrence of charged single ions and neutral ion pairs. Simultaneous analysis of all data revealed that the diffusivity of Rb+ is likely to be lower than that of Na+. Similarly, the diffusivity of RbI0 pairs was found to be smaller than that of NaI0 pairs. Surprisingly, the faster overall transport of Rb as measured by radiotracer diffusion appears to be due to a relatively large fraction of RbI pairs, in conjunction with the finding that the ion pair diffusivities exceed the single cation diffusivities by 2 orders of magnitude.