To better understand the complex equilibria involved in the UNEX process for acidic solvent extraction of radionuclides, the interaction of a carbamoylphosphine oxide ligand (L) with the proton of hydrated chlorinated cobalt(III)dicarbollide acid, H[Co(C₂B₉H ₈Cl₃)₂], has been studied in wet 1,2-dichlorethane (DCE) solution using IR and NMR (¹³C and ³¹P) spectroscopy. The formation of two groups of complexes has been determined. The first group contains three complexes with 1:1 composition of acid to ligand (Scheme 1). The second group of complexes has 1:2 composition in the equilibrium, shown in Scheme 2. Within each group, the complexes differ in composition only by the number of incorporated water molecules. The equilibria (Schemes 1 and 2) are both very sensitive to the content of self-associated water in solution and are driven by its concentration, which is unsteady and depends on the solution preparation history. The simultaneous presence of both anhydrous (I, II) and hydrated (III, IIIa, IV) proton solvates indicates that the enthalpies of carbamoylphosphine oxide complex formation with H ⁺, H₃O⁺, and H₅O₂ ⁺ are very close to each other.