This paper is devoted to the analysis of the mass transfer properties in a grid generated turbulent flow. The closure of the transport equation of the turbulent mass flux has been experimentally investigated. Two combined optical non-intrusive methods, the laser-induced fluorescence and the laser Doppler velocimetry have been used in order to measure simultaneously and instantaneously the concentration of a passive contaminant and the velocity of the flow. It has been demonstrated that the approximation production = pressure scrambling is valid over the major part of the flowfield. A model for the pressure scrambling term has been analysed in order to define a turbulent diffusivity concept.