The structure and growth mode of alkane films (n-CnH2n+2; n = 4, 6, 7) adsorbed on a Ag(111) surface have been investigated by synchrotron X-ray scattering. New models are proposed for the butane (n = 4) and hexane (n = 6) monolayer and butane bilayer structures. Specular reflectivity scans reveal that growth of all films is preempted between two and three layers by nucleation of bulk particles oriented with a single bulk crystal plane parallel to the film. In the case of butane, the bulk particles also have a fixed azimuthal relationship with the film resulting in complete epitaxy.