Tantalum pentoxide thin films on p-type Si substrates were prepared by thermal oxidation at 525-550°C of electron-beam deposited Ta. Polycrystalline Ta2O5 films were formed by heat treating the amorphous Ta2O5/Si structures at temperatures of 900-1000°C for times of up to 2.5 h in Ar or dry O2. The amorphous films crystallized into the low temperature β-Ta 2O5 modification. The electrical properties of the polycrystalline Ta2O5 films on Si were studied employing Al/Ta2O5/Si metal-insulator-semiconductor capacitors. Films which were crystallized in Ar show higher leakage for all fields than comparable amorphous films. Heat treatments (900-1000°C) in dry O 2 for times greater than 30 min produce films with similar leakage at fields lower than 0.6 MV/cm, but increased leakage at higher fields. Increasing the temperature or the length of time of the high temperature oxidation step reduces the leakage. After 30 min at 1000°C in dry O2, a leakage current density of 9.6×10-9 A/cm2 at an applied field of 0.7 MV/cm can be achieved (insulator about 70 nm thick). The conductivity of the dielectric films increases irreversibly (dielectric breakdown) if a current density of 1.93 ×10-4 A/cm2 or higher is forced through the insulator; for a specimen oxidized for 30 min at 1000°C, this corresponds to a breakdown field of about 1.0 MV/cm. The effective dielectric constant εeff of crystallized Ta2O5 films on Si is higher than that of amorphous Ta2O5 on Si and can be as high as 30. For isochronal oxidations at different temperatures greater than about 900°C a decrease of εeff is observed, which increases with increasing oxidation temperature. For isothermal oxidations at high temperatures εeff decreases as a function of oxidation time. Auger electron sputter profiles of high-temperature oxidized Ta2O5/Si structures indicate a thin SiO2 layer at the Ta2O 5/Si interface. The formation kinetics of this SiO2 layer as a function of oxidation time and temperature enables a consistent explanation of all electrical measurement results.