We describe a quantitative ion beam technique for the deposition of compound thin films. The metal atom flux is supplied by inert ion beam sputtering, and the reactive flux is supplied by a low‐energy ion beam directed at the growing film, allowing the fundamental deposition parameters of arrival rates, ion energy, and direction to be measured and controlled. Analysis gives the sputtering yields and incorporation probabilities as a function of film composition, arrival rate ratios, and ion energy. Results are presented for Al films deposited under a range of N2+ ion bombardment (100–500 eV) up to arrival rate ratios exceeding the value needed to form AlN. Nitrogen ions are almost fully incorporated into Al films, and excess N is rejected above the composition N/Al=1. The microstructure is shown to depend on the N/Al arrival rate ratio and the nitrogen ion energy used during deposition.