The structure, mechanical, and chemical properties of boron-aluminum nitride thin films prepared by ion-beam-assisted deposition (IBAD) are reported. The ratio of N to Al + B as determined by Auger electron spectroscopy was consistently 1.0 in all films. The c-lattice constant of the alloy decreased linearly up to approximately 10% BN, suggesting the formation of a single-phase AlN-wurtzite-BN alloy. The hardness of the films was between 20 and 25 GPa, as measured by nanoindentation, and did not change significantly with the amount of boron present. The etch rate in warm phosphoric acid of an alloy containing 9% BN was an order of magnitude lower than that of pure AlN.