The acid-base properties of aluminum surfaces have recently attracted much interest due to their consequences on the adhesion of organic coatings. The present work deals with a novel photoelectrochemical approach to the acid-base properties of aluminum surfaces. Passive films were considered here as wide-gap semiconductors. When they are in contact with an electrolyte, a space charge builds up. A negative (positive) space charge is considered here as a sign of an electron donor (acceptor) surface, i.e., to a Lewis basic (acid) surface. The more negative (positive) the space charge or the corresponding band bending in the semiconductor, the more basic (acid) the passivated surface. The spectrophotoelectrochemical response of the samples polarized at the rest potential under ultraviolet illumination allowed the measurement of the band bending as the difference between the thresholds of two photoemission phenomena from the Fermi level toward the passive film conduction band at the inner and outer interfaces of the passive films. Negative band bendings were always obtained, evidencing the basic character of the analyzed surfaces. A basicity scale was obtained which is compared in Part II to that deduced from X-ray photoelectron and X-ray absorption near-edge structure studies.