A new method for determining the molecular diffusivity of oxygen in liquids is described. The technique was applied through a flat air-liquid interface in a Hele-Shaw cell (5 x 5 x 0.2 cm(3)) and was based on planar laser-induced fluorescence (PLIF) with inhibition. A ruthenium complex (C72H48N8O6Ru) was used as the fluorescent dye sensitive to oxygen. A mathematical analysis was developed to determine the molecular diffusivity of oxygen simply by localizing the gas diffusion front. The specificity of this mathematical analysis is that it does not require the properties of the fluids (such as the saturation concentration) to be considered, which is especially relevant for complex media that are sometimes difficult to characterize properly. This technique was applied to three different fluids (viscosities ranging from 1 to 2.4 mPa center dot s) corresponding to binary diffusion coefficients ranging from 9.5 x 10(-10) to 2 x 10(-9) m(2)/s. Experimental data were found with an uncertainty of about 5% and were in good agreement with the literature. Particle image velocimetry and numerical simulations were also carried out to determine the optimal gas flow rate (0.01 L/s) to reach purely diffusive transfer, and the corresponding hydrodynamic profiles of the two phases. (C) 2012 American Institute of Chemical Engineers AIChE J, 59: 325-333, 2013