Among the various technical options for removing ionic contaminants from wastewaters, membrane processes and especially their coupling with polymer addition have been proven to provide worthwhile prospects for the removal of metal cations. Nevertheless, their use for the removal of anionic pollutants such as oxyanions has been little studied in the literature. In the present work, the rejection of oxyanions forms of Se(IV) and Se(VI) by tight ultrafiltration membranes was deeply investigated under various experimental conditions. This paper aims at understanding the mechanisms governing oxyanion rejection and determining the potential ways to improve performances. It is first shown that selenium concentration and salt content play a leading role on performances, with high selenium rejection even if pores are noticeably larger than ions. Additionally, results reveal that the pH value also has a tremendous impact on performances due to its influence on both membrane and selenium charges. Finally, chitosan addition as a preliminary step was found to have a positive influence on selenium rejection for ceramic ultrafiltration (UF) membranes with a large pore size whereas it had no influence on the performances of organic UF membranes with a lower molecular weight cutoff.