In this paper, a light absorption method is applied to estimate the thickness of an atomizing liquid film at microscale that exits a spraying nozzle. By using a dioxygen sensitive dye, called resazurin, it is also possible to determine the local profile of dioxygen concentration resulting from the gas-liquid transfer from air into the liquid film. The method relies on the use of the well-known Beer-Lambert law, the linearity of which has been experimentally proved for the optical system and the dye concentration used in this study. From thickness and dye concentration measurements in the liquid film formed at the outlet of the nozzle, a mass transfer analysis is performed, providing a local description of the dioxygen mass transfer (concentration fluxes, etc.) occurring in this film. This optical method, leading to a dual measurement of thickness and mass transfer, is the first to be successfully implemented in this type of thin liquid film in sprays (≈ 30 µm) at microscale and with such high velocities (≈ 8 m/s). The method is non-invasive and does not disturb the flow, in comparison with classical liquid phase measurements where the liquid is collected to be titrated. The values of the mass transfer coefficients, ! , deduced from our analysis for the liquid film (2.2 x 10-3-1.2 x 10-2 m/s) are consistent with global measurements already obtained, validating our measurement and giving more insight into the mass transfer occurring in this film.