Properties of interface states at Ta2O5/n-Si interfaces are studied by capacitance–voltage and deep-level transient spectroscopy (DLTS) measurements. The results show that the “slow” states at Ta2O5/n-Si interfaces are not significantly affected by annealing, and their density is about 1.4–1.9×1011 cm−2. The energy distribution of the “fast” interfacial states in the band gap of silicon is obtained, and some high concentration interfacial states are found. DLTS spectra reveal that the interfacial defects at Ta2O5/n-Si interfaces are all donor type. The high concentration interfacial defects are located at about 0.40 eV below the conduction-band minimum of silicon. Their density decreases with increasing annealing temperature. The origin is suggested to be related to the diffusion of tantalum into the Si substrate.