In effort to investigate molecular structures and spectral properties of the selenium trioxide adducts with organic oxygen donor we decided to verify the preparation of the selenium trioxide adducts with 1,4-dioxane. In this reaction was not possible to eliminate reduction of Se(VI) to Se completely due to high melting point of 1,4-dioxane. A white precipitate containing Se(VI) was formed as selenium trioxide dissolved. The Raman spectrum of this product differs substantially from spectra of other donor-acceptor compounds. A glass-like solid was isolated after the solvent removal under vacuum from the filtrate. Sublimation of this glass-like material at 50 cent. deg. under vacuum gave a small amount of cyclic ester C2H4O4Se. Another cyclic ester of selenic acid, C2H2O8Se2.C4H8O2, crystalizes from the reaction mixture after a few days. The reaction of selenium trioxide with 1,4-dioxane in sulfur dioxide was attempted with the aim of isolating the adduct. An equimolar amount of 1,4-dioxane was very slowly added under vigorous stirring to a saturated solution of selenium trioxide in sulfur dioxide at -15 cent. deg. After evaporation of all sulfur dioxide, a white microcrystalline solid was isolated. The content of Se(VI) established by elemental analysis matches the content of selenium in C4H8O2.SeO3. The Raman spectrum of this product corresponds to the spectra of other known selenium trioxide adducts with organic oxygen donors.