A trioxide is a compound with three oxygen atoms. For metals with the M₂O₃ formula there are several common structures. Al₂O₃, Cr₂O₃, Fe₂O₃, and V₂O₃ adopt the corundum structure. Many rare earth oxides adopt the "A-type rare earth structure" which is hexagonal. Several others plus indium oxide adopt the "C-type rare earth structure", also called "bixbyite", which is cubic and related to the fluorite structure.^[1]

• Carbon trioxide, CO₃
• Chromium trioxide, CrO₃
• Molybdenum trioxide, MoO₃
• Rhenium trioxide, ReO₃
• Selenium trioxide, SeO₃
• Sulfur trioxide, SO₃
• Tellurium trioxide, TeO₃
• Tungsten trioxide, WO₃
• Uranium trioxide, UO₃
• Xenon trioxide, XeO₃

• Antimony trioxide, Sb₂O₃
• Arsenic trioxide, As₂O₃
• Bismuth(III) oxide, Bi₂O₃
• Boron trioxide, B₂O₃
• Cobalt(III) oxide, Co₂O₃
• Dichlorine trioxide, Cl₂O₃
• Dinitrogen trioxide, N₂O₃
• Gadolinium oxide, Gd₂O₃
• Gallium(III) oxide, Ga₂O₃
• Gold trioxide, Au₂O₃
• Indium(III) oxide, In₂O₃
• Iron(III) oxide, Fe₂O₃
• Manganese(III) oxide, Mn₂O₃
• Nickel(III) oxide, Ni₂O₃
• Phosphorus trioxide, P₄O₆ (named before the true formula known)
• Thallium(III) oxide, Tl₂O₃
• Terbium(III) oxide, Tb₂O₃
• Trioxidane, H₂O₃
• Vanadium trioxide, V₂O₃
• Ytterbium(III) oxide, Yb₂O₃
• Yttrium(III) oxide, Y₂O₃

• Mineral trioxide aggregate
• Sulfur trioxide pyridine complex, SO₃(py)