In this paper, the application of the diffusion equation to the problem of moisture movement below the fiber saturation point is investigated. The general diffusion equation was solved by numerical methods for the case of a concentration-dependent coefficient and a boundary condition of a specified concentration variation. Experimental concentration-distance curves were generated from the drying of yellow poplar. Solutions to the diffusion equation using coefficients from the literature were then compared with the experimental data. The diffusion coefficient of yellow poplar was determined directly from the concentration-distance curves by the method of Matano. On the basis of the above information, a diffusion coefficient was determined which accurately describes the drying of yellow poplar between 61 and 181 moisture content. / Master of Science