The reaction of chalcogen dioxides ChO₂ (Ch=Se, Te) with As₂O₃ in a 30 molar KOH hydroflux at about 200 °C yielded crystals of potassium trichalcogenides K₂Ch₃ with dimensions up to 2 cm. Arsenic trioxide acts as electron donor and is oxidized to arsenate(V). The new heterochalcogenide anion (TeSe₂)²⁻ formed when starting from SeO₂ and TeO₂ simultaneously. The compound K₂TeSe₂ crystallizes isostructural to K₂S₃ and K₂Se₃. The unexpected redox reaction as well as the precipitation of hygroscopic compounds from an aqueous solution are attributed to a strongly reduced activity of water. The reactions were studied by Raman and UV/Vis spectroscopy. Depending on the concentration of As₂O₃, colorless monochalcogenide Ch²⁻ or orange Se₂²⁻ and purple Te₂²⁻ anions are dominating the solutions.