Abstract--The surface charge density of mica (001) cleavages was determined by counting the number of fission particle tracks in a given area of a 6-ram muscovite disc replica with optical and scanning electron microscopy after saturation of the layer charge by washing with 0.5 M UO2(NO3)2 solution, dilution of the excess salt by washing with 0.01 M UOz(NO3) 2 in 0.005 M HNO 3 (pH 2.4), blotting off the excess liquid, thermal neutron activation in contact with the muscovite disc, etching the muscovite, and counting the 235U fission tracks/cm 2. In initial studies, the uranyl cations were found to hydrolyze from the cleavage surface continuously during the washings with water, ethanol or acetone to remove excess salts, but the urany! cations in the interlayers near broken edges and crystallographical steps were strongly retained even against washings with 0.5 M CaCI2 solution. The hydrolysis of UO~ + from the smooth portions of the flake surfaces was avoided by the use of three 1-hr final washings with the 0.01 M UO2(NO3)2 in 0.005 M HNO3 solution. Each flake was pressed between filter papers three times to remove the excess solution. A negligible amount of excess salt remained on the cover glass controls. The UO~ + cations retained (mean, 3.6 + 0.2 x 10 .7 mequiv./cm 2) on the cleavage surfaces of various micas were nearly equivalent to the theoretical surface charge (cation exchange capacity, 3.5 • 10 .7 mequiv./cm2), showing that hydrolysis was prevented. The uranium on the unblemished mica planar surfaces increased with increasing uranyl concentrations in the final washing solution, indicating that the excess salt remaining on the surfaces had become significant. With a given UO 2+ salt concentration, the uranium on the surface increased on increasing the solution pH from 2.5 to 3.5, attributable to the formation of polymeric ions such as U2025 + and U30~ + with higher uranium retention per unit positive charge equivalent to the fixed negative charge of the mineral surface. Uranyl cations replaced much of the interlayer cations from vermiculites even after K, Rb and Cs presaturation and drying from 110~ were employed. Strong adsorption of uranyl cations (in a form not replaced by washings with a neutral salt solution), which occurred in the defects of micaceous minerals, is important in the interpretation of actinide element retention in soils and sediments wherein these minerals are abundant.