Films of AlN, up to 5 μ in thickness, have been grown by diode reactive sputtering under conditions in which the initial residual gas pressure was less than 10−9 Torr. Measurements of resistivity, capacitance, and dielectric loss were made on film sandwich structures in which the electrodes were sputtered films of tantalum and evaporated films of gold and aluminum. In general, the dielectric properties were superior to those reported for ceramic AlN, with film resistivities in the range 1013 Ω-cm and dissipation factors lower than 0.005. Measurements could be made on samples with tantalum electrodes at temperatures up to 500 °C. The results indicate that the capacitance and dissipation factor show smaller changes with temperature for the sputtered films than for bulk material. Some of the consequences of the film structure in relation to dielectric breakdown and piezoelectric properties are discussed.