Atomistic molecular dynamics simulations are performed to analyze the diffusion of phenol molecules in a bisphenol-A-polycarbonate melt in the zero concentration limit for the temperature range from 500 K to 640 K. The transition from a hopping diffusion to a continuous diffusion is observed. Visualization of the diffusion process reveals a strong coupling between the polymer dynamics, i.e., size and shape fluctuations of the pore space and the hopping of the penetrant. Proper equilibration of the melt has been ensured by application of a novel multiscale simulation approach.