This work proposes an imaging technique that can quantify the diffusion coefficient and the molecular diameter of acetone and diacetyl vapor by following the optical signal given by the phosphorescent emission of these two molecules at low pressures. The technique consists in measuring the diacetyl and acetone diffusion inside a visualization chamber at different nominal pressures and temperatures. These molecules are commonly used as tracers in molecular tagging techniques employed for measuring temperature, pressure and velocity fields in both liquid and gas flows [1,2,3]. Since they are among the simplest molecules in the family of ketones, many chemistry oriented researchers have studied the internal molecular structure of acetone and diacetyl by measuring the spontaneous emission that follows a photon excitation in the absorption spectrum [4,5]. However, there is still some controversy in the literature regarding the value of kinetic properties such as the self-diffusion coefficient and the kinetic molecular diameter of acetone and diacetyl [5,6,7]. An accurate estimation of these kinetic properties is of significant importance for both (i) correctly applying the molecular tagging technique to gas flows and (ii) predicting the macroscopic behavior of the gas-tracer mixture flows by using numerical models of continuum or molecular natures. Therefore, further experimental data on acetone and diacetyl kinetic properties are required. The diffusion coefficient is measured by tracking with a CCD camera the spreading distribution of light emitted by the excited molecules. The system evolves from a compact emitting beam at first excitation towards a more chaotic agglomeration of molecules at the end of the experiment. Figure 1 represents the diacetyl diffusion at 0.5 µs and 300 µs after laser excitation. From the diffusion data, the Chapman-Enskog theory and a suitable collision model can provide an estimation of the molecular diameter of acetone and diacetyl molecules.