Aluminium-titanium alloy coatings are attractive materials for a wide range of applications from aerospace and high performance engine applications to solar absorbers, depending on the alloy deposited and its corresponding properties. Consequently, a detailed study of the Al-Ti alloy system has been carried out, and the results to date are presented here. In this study, Al-Ti films have been prepared by physical vapour deposition using two planar unbalanced magnetrons in a closed field configuration, with a biased substrate. Pure aluminium and pure titanium sputtering targets were used for the co-deposition of coatings with compositions varying throughout the full binary range from pure aluminium to pure titanium. The coatings were deposited onto glass substrates. The deposited films were analysed by EDX, XRD and SEM processes in the as-deposited state and after high temperature annealing, to investigate changes in structure and phase composition. The as-deposited coatings contain only aluminium and titanium phases but after the annealing process, stable intermetallic phases are formed. Attempts have been made to relate these changes to specific process and geometrical factors within the deposition system. The tribological properties of these coatings were reported in a companion paper (The tribological properties of co-deposited aluminium-titanium alloy coatings, presented at ICMCTF 2003, San Diego, April 28th-May 2nd, 2003). (C) 2003 Elsevier Science B.V. All rights reserved.