Self nucleation layers, nominally 50 nm in thickness, have been predeposited at temperatures in the range 600–1100°C and shown to markedly and uniformly improve the surface morphology of epitaxial layers of GaN on highly oriented (0001) sapphire substrates. However, the epitaxial relationship between the GaN film and the sapphire substrate and the electrical and optical properties of these films depend strongly on the temperature at which the nucleation layer is deposited. For example, growth on a nucleation layer deposited at 1100°C yields a film little changed In carrier concentration or mobility and exhibiting the epitaxial relationship [(O0 0 1)GaN// (O001)sapphire and (1010)GaN //(1120)sapphire] characteristic of films grown directly onto (0001) sapphire. In contrasT, film growth onto a nucleation layer deposited at 600°C results in a typically oriented but semi-insulating layer. Of particular interest, an intermediate growth temperature (850°C) yields GaN films that are unconventionally twinned and semiinsulating. The twinning is distinguished by GaN sublattices that are rotated ±10.9° with respect to the sapphire lattice, such that (3030)GaN //(4150)sapphire. While all films in this regime are twinned and highly resistive, examples of scattering from twins with unequal volumes and non-ideal rotation angle have been occasionally encountered. The results clearly demonstrate that the orientation of the nucleation layer governs the orientation and properties of the final film.