In the design of waste disposal facilities and evaluation of groundwater and soil contamination by organic and inorganic species, it is essential a knowledge of the mechanisms of transport of the contaminants. There are two basic processes in the transport of solutes: advection and hydrodynamic dispersion.Advection is the process by which moving groundwater carries with it dissolved solutes. The amount of solute being transported depends on its concentration in the groundwater, on the volume of groundwater flowing, and on the groundwater velocity. The hydrodynamic dispersion mechanism involves mechanical dispersion and diffusion. As solutes are carried through porous media, the process of dispersion acts to dilute the solute and reduce its concentration. Diffusion, is the process by which both ionic and molecular species dissolved in water move from areas of higher concentration to areas of lower concentration, even in the absence of groundwater flow.Advective-diffusive transport through unsaturated media is somewhat more complicated than for saturated soils. In this case, the transport parameters may vary both in space and in time. Hence, the determination of such parameters in laboratory should be performed as close as possible to in situ conditions.The processes by which the substances move through porous media can be expressed through mathematical models. They are based on the integration of the advection-dispersion equation using specific boundary conditions for the system of interest. However, their applicability has been often limited due to the lack of reliable parameters obtained experimentally. This thesis seeks to fulfil the gap between numerical modelling and experimental studies. A new diffusimeter has been developed to measure the diffusion coefficient in unsaturated soils. It combines the single reservoir method, proposed by Rowe et al. (1988), with the osmotic control technique. Tests were performed under suction values ranging from 0 to 500kPa. Sodium chloride and potassium chloride were used as solutes diffusing through a compacted inert sample prepared in the laboratory. The results obtained indicated that the new proposed technique worked quite well, being however recommended an improvement in the system of suction control through the use of more precise instrumentation.