An IR spectroscopic study has established that in water-dichloroethane equilibrium solutions carbamoylphosphine oxide forms only one complex, R′R₂P=O⋯HNO₃, with HNO₃, whereas with HClO₄ it forms a number of solvation-separated ion pairs (IP). The structure and composition of the cation moiety of these IP depend on the molar ratio C_CMPO/C_HClO4 and the solution preparation temperature. A CMPO:HClO₄ = 2:1 complex is formed when C _CMPO/C_HClO4 > 2; for C_CMPO/C_HClO4 < 2, 1:1 and 1:2 complexes and a complex with HClO₄ attached to the tertiary nitrogen of CMPO are formed. A detailed discussion is given for all complexes. It is shown for the first time that H⁺ can interact with three oxygen atoms of two P=O groups and one C=O group (or a water molecule) at once. In the absence of CMPO, dichloroethane extracts HNO₃ in the form of micelle-like associates, whose nucleus has a solubilized HNO₃ molecule in the form of a solvation-separated IP.