Reactions of HgII‐, TeIV‐, and SeVI‐containing starting materials under moderate hydrothermal conditions (230 °C, one week) resulted in the formation of the mercuric oxotellurate(IV)/oxoselenate(VI) phases Hg3SeTe2O10 and Hg3SeTe4O14. The mixed‐valent phase Hg5Se2O8 containing mercury in oxidation states +I and +II as well as selenium in oxidation states +IV and +VI also formed under these conditions. The crystal structures of the three compounds were determined from single‐crystal X‐ray data. All structures contain building blocks characteristic for HgI or HgII cations and for oxochalcogenate anions of SeIV, SeVI, or TeIV. Whereas the oxoselenate(VI) and oxotellurate(IV) anions are isolated in the structure of Hg3SeTe2O10, the higher TeO2 content in the structure of Hg3SeTe4O14 leads to a condensation of the corresponding anions. The crystal structure of Hg5Se2O8 is described in terms of oxo‐centered [OHg4] tetrahedra.