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Flat-plate streaming potential measurements were carried out on samples of tantalum pentoxide and silicon dioxide. The Ta₂O₅ samples were 150 nm films on silicon substrates, fabricated by the deposition of tantalum followed by oxidation in pure oxygen at 550°C. The SiO₂ samples were 1 μm thick films prepared by steam oxidation of a silicon substrate. The electrolyte used in the measurements was NaCl, at concentrations of 10⁻¹, 10⁻², and 10⁻³ M. The point of zero zeta potential (pH_iep) was between 2.7 and 3.0 for the Ta₂O₅ samples and between 2.6 and 3.2 for SiO₂. In both cases, this agrees well with previous literature determinations, and indicates the reliability of the flat-plate method. We observed that the zeta potential of thin films of Ta₂O₅ was more stable and had less hysteresis than the zeta potential of thin SiO₂ films.