We investigated the photoluminescence of Eu-doped ZnO and ZnGaxO1 + 1.5x (x = 1.8–2.8) films upon bandgap excitation to identify the occupation sites of Eu3+ ions through varying the host crystal material, substrate material, and reactant gas. Eu3+ emissions from ZnGaxO1 + 1.5x:Eu films deposited on Si and SiO2 substrates had similar spectral shapes, but their intensities were different, correlated with different defect emission levels. When depositions were carried out in H2O gas, the Eu3+ emissions from the c-axis oriented ZnO:Eu films consisted of a very sharp main peak (P1) at 612 nm and a sub-peak (P2) at 619 nm for 5D0 → 7F2 transition, whereas those from randomly oriented polycrystalline ZnGaxO1 + 1.5x:Eu films were broader and could be divided into four components (P1, P2, P3, and P4), of which the wavelengths of P1 and P2 matched those of the ZnO:Eu films. The availability of both Ga3+ and Zn2+ sites in the ZnGa2O4 lattice expands the variance of chemically distinct sites that can be occupied with Eu3+ compared with only divalent Zn2+ sites in the ZnO crystal. Films deposited with O2 exhibited lower emission intensities with a narrower spectrum width than those deposited with H2O. This behavior was correlated with the X-ray diffraction peak width of ZnGa2O4(311). High-quality crystal lattices formed with O2 allowed only a few sites to accommodate Eu3+ ions. In contrast, OH− and H+ derived from H2O will modify crystallinity to generate various emission-active sites, resulting in intense and broad emissions. Possible sites for the four components are discussed.