Summary: Properties and possible structures of hydronium trihydrate H₉O₄⁺ and hexahydrate H₁₅O₇⁺ are considered. From the 2330 minerals listed in Strunz's mineralogical tables nineteen were selected that appear to contain H⁺.4H₂O or H⁺.7H₂O, and the possibility of discrete H₉O₄⁺ or H₁₅O₇⁺ groupings existing in their crystal lattice is discussed. From a crystallochemical point of view the most probable examples of hydronium hydrate compounds are layer lattice minerals like H-montmorillonite, H-vermiculite, troegerite, H-meta-autunite, sabugalite, and hewettite.