Interaction of hydrated proton, H₅O₂ ⁺·(H₂O)₄, in dichloroethane solutions with diphosphine dioxides (L) having methyl (Ph₄Me), ethyl (Ph ₄Et) and polyoxyethylene chains (Ph₄PEG) linking two diphenyl phosphine oxide groups has been investigated. A bulky counter ion: chlorinated cobalt(III) bis(dicarbollide), [Co(C₂B₉H ₈Cl₃)₂]^-, minimizes perturbation of the cation. At low concentrations, Ph₄Et and Ph₄PEG form anhydrous 1:1 complexes with (P)O-H⁺-O(P) fragment having very strong symmetrical H-bonds. At these conditions Ph₄Me form another compound, H₅O₂⁺·L(H₂O) ₂, due to lower PO basicity and optimal geometry of the chelate cycle. At higher concentrations, Ph₄Me and Ph₄Et form isostructural complexes H₅O₂⁺·L ₂, whereas Ph₄PEG forms only a 1:1 complex with proton dihydrate, H₃O⁺·H₂O. In excess of free Ph₄Me and Ph₄Et a water molecule is introduced to the first coordination sphere of H₅O₂⁺ and the average molar ratio L/H₅O₂⁺ of the complexes exceeds 2. The composition of these complexes as a function of L and its concentration is discussed.