Engineered fills, glacial tills, mudflows, debris flows, residual soils, and colluvial deposits have a structure consisting of a soil matrix (e.g. sand) and large dispersed particles (e.g. gravel) mixed in the matrix. Mixtures of this type have received little attention in soil mechanics. The purpose of this study was to evaluate the effect of dispersed particles on the permeability of sand-gravel mixtures. Constant head permeability tests (ASTM D 2434-68) were conducted on samples having 63.5 mm in diameter and 155 mm in height. The samples were made of a matrix of Ottawa sand (d_ave = 0.725 mm) and dispersed particles with an average diameter of 11.1 mm. The percentage by volume of gravel in the mixtures was varied between 0% and 14%. The results indicated that the permeability of the mixture decreased as the volume percentage, B, of gravel in the matrix increased. It was determined that the permeability of the mixture, K_m, can be obtained from the permeability of the sand matrix, K_s, and the percentage by volume, B, of the gravel in the mixture by using the relationship: K_m=K_s[(1−B)/(1+B/2)]. Only K_s of the sand matrix and the percentage by volume, B, of the dispersed gravel need to be known in order to obtain the permeability of the mixture, K_m. The effect of clusters of dispersed particles in the sample was also investigated. It was found that the permeability of mixtures with clusters depends on the relative location of the clusters within the mixture.