Thin films of Pb(MgxNb1−x)Oz(hereafter called PMN) have been deposited on carbon substrate by reactive ion beam sputtering, from a mixed oxide–metal multicomponent target composed of magnesium oxide, and metallic niobium and lead. The substrate temperature and sputtering gas composition (oxygen pressure during deposition) were varied and their effects on the film composition and compositional uniformity were investigated. The major effect seen was the systematic variation in Pb content with changes in temperature and gas composition. We also observed a spatial variation in Pb composition across the substrate and believe this to be caused by film bombardment by argon ions specularly reflected by the target surface. This was supported by results showing a systematic reduction in Pb content in films subjected to secondary ion bombardment during film growth. The selective Pb depletion in PMN films is discussed in terms of various mechanisms found in reactive sputtering environments.