Diffusion of a metal (Cu) into a fully cured polymer (polyimide) below the glass transition has been systematically investigated for the first time by means of low-energy ion sputtering in combination with a radiotracer technique. The penetration profiles exhibit a steep non-Fickian decay of the tracer concentration over several orders of magnitude followed by a relatively flat Fickian tail. The initial decay is attributed to the formation of Cu clusters and is discussed in terms of a Monte Carlo simulation. The Fickian tail is related to the diffusion of single Cu atoms. Diffusion measured at the lowest temperature deviates from linearity on the Arrhenius plot and seems to be controlled by a free-volume mechanism of diffusion below the glass transition temperature.