The synthesis of compound thin films by dual ion beam deposition is described. In Part I of this two-part paper, the experimental approach is presented, together with an analysis of composition for the aluminum-nitrogen system. In the accompanying paper (Part II), the properties of Al-N films are described. The dual beam technique supplies a deposition flux of Al from Ar + ion beam sputtering of an Al target. Simultaneously, a low-energy (100-500 eV) N+2 ion beam bombards the growing film. Using a measured ion beam gradient across the substrate holder, a wide range of arrival rate ratios (0≤N/Al≤1.6) is obtained in each deposition run, producing a composition range of (0.1≤N/Al≤1.0). The film composition varies linearly with N arrival flux and saturates at the composition of stoichiometric AlN. The incorporation probabilities of Al and N are close to unity for N/Al≊0, and decrease to 0.7 at N/Al=1.0. Preferential sputtering of Al occurs for N/Al<1, with ejection of excess N for N/Al>1. We discuss the accuracy of this technique in establishing quantitative deposition parameters for compound and composite thin films.