Although recent studies have detected elevated concentrations of arsenic and uranium in inland, alkaline, saline lakes, the chemical reactions associated with these enrichments are poorly understood. In this study, we used a five-year study of water chemistry, characterization of arsenic and uranium in sequentially extracted sediments, and X-ray absorption fine structure analyses (XANES) to examine the geochemical behavior of arsenic and uranium during evaporation of Orog Lake, a saline-alkaline lake in the arid region of Mongolia. Water chemistry analyses showed that the concentrations of arsenic and uranium increased as evaporation caused the lake to shrink. Comparisons of monitoring results with predictions of a geochemical model suggested that some of the arsenic was removed from the lake water under highly desiccated conditions. Sequential extraction and XANES analyses showed that As (V) could be removed by ferrihydrite, even at pHs greater than~ 9. In contrast, the accumulation of uranium in the lake could be reproduced by consideration of only evaporation. The conservative behavior of uranium could be explained by the low affinity of U (VI) for carbonate and ferrihydrite at pHs> 9 and high concentrations of dissolved inorganic carbon. This study provided evidence of conservative accumulation of U in an alkaline lake.