We have used transition-state theory to estimate diffusion-energy barriers and to determine the characteristic mechanisms by which n-butane diffuses on model Pt(lll) and Cu(001) surfaces. Our studies reveal some of the unique features associated with molecular mobility, including one-jump mechanisms for molecular hopping between non-nearest-neighbor binding sites; conformational correlation in molecular hopping; short-ranged, directional anisotropy in diffusion induced by the molecular degrees of freedom; and the role of multiple-binding sites in various hopping mechanisms. We discuss some experimental ramifications of these unique features.