In this study, a Y target was sputtered in radio frequency (rf)-excited, rare gas discharges (Ne, Ar) containing 0%–40% O2, operated at cathode voltage from −1.0 to −1.7 kV. In situ optical emission spectrometry was used to monitor two neutral excited Y atom transitions (λ=0.6191, 0.6793 μm) and an excited O atom transition (λ=0.7774 μm) as a function of changing process parameter. Films were grown on fused SiO2 substrates, and their crystallography, optical behavior, and electrical resistivity was determined. A ‘‘phase diagram’’ for Y–O not grown under conditions of equilibrium thermodynamics was constructed, and included hexagonal Y, cubic Y2O3, and Y and Y2O3 that had no long range crystallographic order. Two direct optical transitions across the energy band gap of cubic Y2O3, at 5.07 and 5.73 eV, were identified. Combining discharge diagnostics, growth rate, and film property results, it was concluded that Y2O3 was formed at the substrate concurrent with the complete oxidation of the target surface. Even after target oxidation, the discharge contained atomic Y. On the basis of fundamental optical absorption edge characteristics, cubic Y2O3 that more closely resembled the bulk material was obtained when the Y-oxide molecule/Y atom flux to the substrate was high.