This paper presents a unified testing approach for measuring drying and wetting diffusion coefficients for unsaturated soil in the laboratory. Transient moisture flow in an unsaturated soil in response to suction changes is controlled by the unsaturated moisture diffusion coefficient. The moisture diffusion coefficient can be determined by measuring suction profiles over time. The laboratory testing approach involves measurement of the total suction changes with time with thermocouple psychrometers in cylindrical soil specimens with predetermined boundary conditions. The determination of the diffusion coefficient by this method is simple and relatively rapid and can be carried out on a routine basis in a geotechnical engineering laboratory. It is well-known that soils exhibit hysteresis with drying and wetting cycles. A similar hysteresis has been observed between the drying and wetting diffusion parameters for a number of undisturbed soil samples obtained from field borings across Oklahoma. The wetting diffusion coefficients are generally greater than the drying diffusion coefficients by a factor of up to 2.