CdTe epitaxial films have been grown on GaAs on (100) silicon by congruent evaporation on CdTe in ultrahigh vacuum. The (100)CdTe orientation is strongly preferred in growth on (100)GaAs/Si, in contrast with the (111)CdTe preference on (100)GaAs substrates. Growth of the (111) orientation requires special processing. The (100)GaAs epilayer grows on misoriented (100)Si with a lattice tilt of ∼0.04° (back toward normal) for each degree of silicon misorientation. A similar lattice tilt in CdTe growth, in combination with the variable misorientation of the three-dimensional as-grown GaAs epilayer, is probably responsible for a mosaic structure in CdTe epilayers on GaAs/Si. In (111)CdTe layers, rotational twinning can occur on unpolished GaAs/Si. With increasing GaAs/Si misorientations, increasing numbers of (100) twin inclusions can also be found. Chemical polishing prior to CdTe growth eliminates the mosaic and twinning problems in most cases.