The electronic properties of mixed BaSr oxides were investigated both spectroscopically and by electrical measurements in MOS-diode structures on Si (001). The dielectric properties of crystalline Ba 0.7 Sr 0.3 O turned out to be very good with a dielectric constant of ∊=21.8±0.2, interface trap densities 6–9×10 10 eV −1 cm −2 near midgap, negligible capacitance–voltage hystereses smaller than 10mV, low leakage currents (below 2mA/cm −2 at 5nm oxide thickness), and band offsets of 1eV for both conduction and valence bands. At the smallest oxide thickness of 5nm a capacitance equivalent thickness (CET) of 0.75nm was achieved. Effective flatband voltages could be tuned by inserting an ultrathin Al layer between metal gate and oxide. Above 400°C the oxide is transformed into a silicate of defined stoichiometry.