Erbium-doped yttrium-aluminum oxide films (Er:Y2O3-Al2O3) are deposited by aerosol-assisted metal-organic (AA-MO)CVD. The effects of the humidity of the carrier gas and UV assistance on their structure and optical properties during the deposition are investigated as a function of the substrate temperature and the aluminum mole fraction (Al2O3 mol.-%) in the liquid solution. The effect of substrate temperature is studied for a constant Al concentration of 33.33 mol.-% of Al-acac in the solution. The maximum deposition rates are reached under lower air humidity and with UV assistance in a surface temperature range between 350 and 460 °C. Nevertheless, as-deposited Er:Al2O3-Y2O3 films show a very low organic contamination when depositions take place under high air humidity and with UV assistance. The film composition is strongly dependent on air humidity, showing a very high aluminum content when working with a high humidity of the carrier gas, and yttrium-rich when working with a low humidity of the carrier gas. The refractive index of Er:Al2O3-Y2O3 films under these conditions is relatively high, reaching 1.76 when deposited at 460 °C. The effect of composition is studied at a substrate temperature of 410 °C. The effect on film composition when varying the aluminum mole fraction in the liquid solution is studied. The most influential parameter is the high air humidity, which induces stronger variation on the layer composition for the same liquid composition.