Fine sediments usually have a much greater capacity to sorb metals and radionuclides than coarse sediments. In some estuarine sediments studied here the cation binding capacity of the sand fraction was much larger than would be predicted from estimates of specific surface area. This anomaly is shown to be due in part to surface coatings of iron compounds and to clay particles cemented to the sand grains by these coatings. Treatment of the sands with ultrasound and 0.5 M HCl removes these surface coatings and reduces their capacity to sorb Cs by a factor of four, but has a much smaller impact on their ability to sorb Zn and Co. Even after the removal of surface coatings the difference in partition coefficients between sands and clays was much less than expected.