<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>ThB<sub>4</sub>O<sub>8</sub> and UB<sub>4</sub>O<sub>8</sub> are synthesized from the corresponding metal oxides and B<sub>2</sub>O<sub>3</sub> at 5.5 and 10.5 GPa, respectively (multianvil Walker‐type apparatus, 1100 °C, 10 min).</p> </abstract>