We have studied the morphology of pentacene films on different silicon surfaces with orientations between Si(1 1 1) and Si(0 0 1). Pentacene molecules are immobile on the Si surface, covering and passivating reactive defect sites. Molecules deposited on top of a closed amorphous wetting layer exhibit isotropic diffusion with diffusion lengths of several tens of micrometers. Neither steps, nor the surface structure affect the film morphology. The wetting layer thus acts as an interfactant, and isolates the molecular film from the substrate, explaining the high quality of pentacene films.