The environmental mobility of newly deposited radionuclides in surface soil is driven by complex biogeochemical relationships, which have significant impacts on transport pathways. The partition coefficient (K d) is useful for characterizing the soil-solution exchange kinetics and is an important factor for predicting relative amounts of a radionuclide transported to groundwater compared to that remaining on soil surfaces and thus available for transport through erosion processes. Measurements of K d for 238 U are particularly useful because of the extensive use of 238 U in military applications and associated testing, such as done at Los Alamos National Laboratory (LANL). Site-specific measurements of K d for 238 U are needed because K d is highly dependent on local soil conditions and also on the fine soil fraction because 238 U concentrates onto smaller soil particles, such as clays and soil organic material, which are most susceptible to wind erosion and contribute to inhalation exposure in off-site populations. We measured K d for uranium in soils from two neighboring semiarid forest sites at LANL using a US Environmental Protection Agency (EPA)-based protocol for both whole soil and the fine soil fraction (diameters< 45 μm). The 7-d K d values, which are those specified in the EPA protocol, ranged from 276–508 mL g− 1 for whole soil and from 615–2249 mL g− 1 for the fine soil fraction. Unexpectedly, the 30-d K d values, measured to test for soil-solution exchange equilibrium, were more than two times the 7-d values. Rates of adsorption of 238 U to soil from solution were derived using a 2-component (FAST and SLOW) exponential model. We found significant differences in K d values among LANL sampling sites, between whole and fine soils, and between 7-d and 30-d K d measurements. The significant variation in soil-solution exchange kinetics among the soils and soil sizes promotes the use of site-specific data for estimates of environmental transport rates and suggests possible differences in desorption rates from soil to solution (eg, into groundwater or lung fluid). We also explore potential relationships between wind erosion, soil characteristics, and K d values. Combined, our results highlight the need for a better mechanistic understanding of soil-solution partitioning kinetics for accurate risk assessment. Health Phys. 93 (1): 36–46; 2007