A collection of six decationized eudialytes from alkaline massifs of various regions was investigated by the single-crystal method. These samples differ from typical eudialytes by their low contents of alkaline and alkaline-earth cations and the high degree of hydration. The process of eudialyte decationization is accompanied by hydration, and hydrous species are incorporated into the mineral structure predominantly as H3O-groups. Those oxonium groups act as monovalent cations, replacing alkaline and alkaline-earth cations and compensating the positive charge deficiency. In all N-positions, H3O-groups replace large cations either partially or completely. Another specific feature of these samples is their low symmetry R3, which is rare compared to R3m in eudialytes.