The diffusivity concept remains up to the present time rather poorly defined physically. Indeed, the dependency of the diffusion coefficient upon the fluid content of the unsaturated porous medium is not clearly established yet.This paper presents an experimental approach of this dependency. The diffusivity coefficientD(&thetas;) has been measured at different water content &thetas; in horizontal columns of glass beads. An analysis of the method of measurement ofD(&thetas;) shows that simple relationships betweenDand &thetas; can be obtained within well-defined water content ranges. The phenomena occurring in the dry range of water content seem basically different from those in the wet range. Hence, the study of unsaturated flow through porous media is somehow to be reexamined in the whole range of water contents.