Uranium L-III-edge X-ray absorption spectroscopy is often used to probe the oxidation state and coordination of uranium in environmental samples, and micrometre-sized beams can be used to spatially map the distribution of uranium relative to other elements. Here a variety of uranium-containing environmental samples are analyzed at both microbeam and larger beam sizes to determine whether reoxidation of U(IV) occurred. Monomeric U(IV), a recently discovered product of U(VI) reduction by microbes and certain iron-bearing minerals at uranium-contaminated field sites, was found to be reoxidized during microbeam (3 mu m x 2 mu m) analysis of biomass and sediments containing the species but not at larger beam sizes. Thus, care must be taken when using X-ray microprobes to analyze samples containing monomeric U(IV).