The deposition of ultra thin films of Al on TiO2 (100) rutile has been studied in UHV using scanning tunneling microscopy. Annealing of the (100) surface to temperatures around 1000-degrees-C produces a stable, microfaceted morphology with the facets oriented along the c axis. Aluminum deposited at room temperature appears to grow in islands, in contrast to observations on (110). Tunneling spectroscopy indicates the nature of the deposited material to be metallic. After annealing, the submonolayer Al film interacts with the surface of the TiO2 by reconstruction. This may indicate a thermodynamic energy barrier to the incorporation of the Al into the cation terminated (100) surface.