The results of in vacuo x-ray photoelectron spectroscopy and Auger electron spectroscopy of Al-diamond interfaces for Al overlayer thicknesses up to 10 Å are presented. Postdeposition annealing effects up to 430°C are also discussed. Ex situ current-voltage (I-V) measurements were also made on thick (∼1500 Å) Al contacts on diamonds. The as-grown diamond surface, on which Al was a rectifying contact, did not chemically interact with Al, even after annealing. An Ar+-sputtered diamond surface, on the other hand, did react with Al to form Al - C bonds upon annealing at temperatures as low as 430°C. Al on the sputtered surface resulted in an ohmic contact. The distortion of the diamond network and formation of vacancies and unsatisfied bonds via Ar+ sputtering of the diamond surface change the I-V characteristics of the Al contact and also facilitate the solid state interdiffusion of Al and C as well as interface reactions at elevated temperatures.