The reaction of (SeO3)(4) with 1,4-dioxane (diox, dioxane) with or without diluting solvent led to the isolation of the unprecedented esters of selenic acid-1,2-ethyl selenate (CH2O)(2)SeO2 and the glyoxal diselenate O2Se[(OCHO)(2)]SeO2. It was possible to isolate an unknown dimeric form of Se2O5 (Se4O10-(diox)(2)) and a geometrical isomer of the mixed-valence oxide trans-Se3O7, both stabilized by dioxane. The dioxane adduct of monomeric selenium trioxide SeO3-diox was obtained from the reaction of (SeO3)(4) with dioxane in liquid SO2. The reaction mechanism for the formation of these compounds was elucidated, and the molecular structure of the unstable form of the selenium trioxide was determined, consisting in a trimeric arrangement (SeO3)(3).