Solid-solution Al-Ta alloys possess significantly higher pitting potentials than pure aluminum in Cl- solutions. Dynamic imaging microellipsometry was used to study the passive film formation on solid solution Al-Ta alloys. Film thickness was measured during growth for alloy compositions of pure Al, Al-8 a/o Ta, Al-25 a/o Ta, and pure Ta at applied potentials of 0.0, 1.0, 2.0, and 5.0 V SCE in a pH 7.2 borate buffer. Increasing the concentration of tantalum resulted in the formation of thinner passive films at all applied potentials. On pure Al and Al-8 a/o Ta, precipitation of an Al(OH)3 type film results in a linearly increasing film thickness with time. The relationship of film thickness measurements to the enhanced localized corrosion resistance is discussed.