We have used transition-state theory to study the diffusion of short n-alkanes (n-butane-n-decane) on a model Pt(111) surface. We have determined diffusion energy barriers, preexponential factors, and characteristic mechanisms by which these molecules diffuse. Our studies reveal novel features associated with molecular mobility, including: Non-nearest-neighbor hops, local minima, conformational correlations, and directional anisotropy induced by molecular orientation We examine factors that contribute to the relationship between molecular size and mobility. Tracer-diffusion coefficients for these molecules are best described by a heterogeneous lattice model. Diffusion coefficients calculated using this model agree well with those from experimental studies.