The effective permittivity of an unconsolidated porous material is shown to be not only a function of the permittivity of the individual components but also of the distribution of particle sizes present. Increasing width of particle size distribution for glass spheres and sieved quartz sand is demonstrated to reduce the effective permittivity of a mixture compared to the effective permittivity of the corresponding monosize material. The results are shown to lie between predictions made using the Maxwell-Garnett <1904> formula for monosize spheres and a self-similar model <Sen et al., 1981> and can be predicted by a multiple Maxwell-Garnett-based mixing model. The results demonstrate that the degree of sorting of a material will directly influence the effective permittivity of the mixture. This is beyond the effect of the change in permittivity caused by the significant change in porosity resulting from the mixing of different particle sizes. ¿ 2001 American Geophysical Union