The moisture diffusion properties of unsaturated soils control the rate of infiltration of surface moisture into the soil mass and hence are critical to a wide variety of civil structures, including pavements, structures, retaining walls, and slopes. Because of the dependence of permeability on suction and the nonlinearity of the suction-moisture relationship, the analytical formulation for flow through unsaturated soils is highly nonlinear. An approximate linear analysis of this problem, which was originally proposed by Peter Mitchell, was investigated. One advantage of this approximate analysis is that it can provide the practical basis for measuring soil moisture diffusion characteristics in laboratory tests. A second advantage is that the linear formulation provides an analytical tool accessible to practitioners. Mitchell originally based his formulation on a relatively restrictive assumption on the permeability-versus-suction relationship. An approach to circumventing that restriction is proposed. The findings of a laboratory test program that uses Mitchell's formulation to estimate a soil's moisture diffusion characteristics are presented. Finally, some simple analytical predictions demonstrate the practical significance of the soil moisture diffusion properties.