Aluminium and their alloys readily react with air to form surface oxides and films. If they become entrained in a casting, they form double oxide film defects that reduce the mechanical properties of the component. It is important to determine the nature and behaviour of the films formed, so that defects can be avoided or their internal gas volumes reduced. The reaction of common additions and impurities that may be present in aluminium alloys were melted at 750 °C in air and nitrogen, to determine thickness, composition and structure of possible films that may form in a defect. The effects of molybdenum were also investigated. It has also been suggested that oxides formed on aluminium alloys may act as heterogeneous nucleation points for solidification. Spinel containing copper layers were manufactured by electro-codeposition, and added to commercial purity aluminium to test its potential as a grain refiner. It was compared with non-refined aluminium and aluminium refined with standard Tibor refiner. The lattice parameters of oxides formed were also compared with that of aluminium to ascertain their potential as nucleation centres. The results are discussed in this work.