Many suboxic sediments and soils contain an Fe(II,III) oxide called green rust. Spectroscopic evidence showed that selenium reduces from an oxidation state of +VI to 0 in the presence of green rust at rates comparable with those found in sediments, Selenium speciation was different in solid and aqueous phases. These redox reactions represent an abiotic pathway far selenium cycling in natural environments, which has previously been considered to be mediated principally by microorganisms. Similar green rust-mediated abiotic redox reactions are likely to be involved in the mobility elf several other trace elements and contaminants in the environment.