Chemical-based technologies for uranium enrichment have been investigated. These make use of a slight preference for higher oxidation states shown by 235U compounds compared to 238U. A process investigated in France uses solvent extraction to preferentially discriminate between compounds in the IV and VI oxidation states, whilst a process developed in Japan uses solid phase ion exchange to preferentially discriminate between the III and IV oxidations states. Both processes require many stages to achieve appropriate levels of enrichment and neither has been developed to commercial operation. A thermal diffusion process was briefly employed during the Manhattan Project but soon abandoned, while plasma separation methods have been studied in the USA and France.