Systematic measurements of the longitudinal dispersion coefficient, DL, were made in 10 different soil samples. Four samples consisted of uniform grains and six non-uniform, composed by mixing the uniform samples. Tests yielded a linear relationship between DLand the seepage velocity, u, for both uniform and non-uniform samples. The process of dispersion, however, has a complex character in non-uniform media, particularly in samples with a discontinuous grain-size distribution curve. In such cases the value of DLmay increase up to the value of DLfor the coarser-grained component depending on the deviation of the average grain size of the individual components. These irregular dispersion properties probably arise from the irregular pore-size distribution of the non-uniform samples.