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Abstract: The effect of dissolved organic matter (DOM) on Am(III), Pu(IV), Np(V), and U(VI) sorption was investigated with natural water (pH ∼8) and zeolitized tuff samples collected from the Rainier Mesa tunnel system, Nevada Test Site, where the USA detonated underground nuclear tests prior to 1992. Perched vadose zone water at Rainier Mesa has high levels of DOM as a result of microbial degradation of mining debris (diesel, wood, etc.). The Am and Pu sorption Kds were up to two orders of magnitude lower in water with high DOM (15–19mg C/L) compared to the same water with DOM removed (<0.4mg C/L) or in naturally low DOM (0.2mg C/L) groundwater. In contrast, Kds of Np and U were less affected by DOM at these solution conditions. Uranium sorption decreased as a result of high dissolved inorganic C (DIC) resulting from microbial degradation of DOM. Thermodynamic model predictions, based on actinide–humic acid stability constants available in the literature, are in general agreement with measured K d data, correctly predicting the effects of DIC and DOM on actinide retardation. This agreement is encouraging to future modeling efforts and suggests that effects of DOM and DIC can be incorporated into reactive transport modeling predictions. The Am and Pu transport rates in Rainier Mesa tunnel waters will be substantially faster as a result of the elevated DOM levels. Low diffusion rates of actinide–DOM macromolecular complexes may focus Pu and Am transport into fractures and minimize retardation via matrix diffusion. The resulting transport behavior will affect actinide distribution patterns and associated risk estimates. [Copyright &y& Elsevier]