The process of protonation of [2,6-B₁₀ H₈ O₂ CCH₃ ]⁻ was investigated both theoretically and experimentally. The most suitable conditions for protonation of the derivative [2,6-B₁₀ H₈ O₂ CCH₃ ]⁻ were found. The process of protonation was carried out in the presence of an excess of trifluo-romethanesulfonic acid CF₃ SO₃ H at room temperature in dichloromethane solution. The structure of the resulting complex [2,6-B₁₀ H₈ O₂ CCH₃ *H^fac ]⁰ was established using NMR data and the results of DFT calculations. An additional proton atom H^fac was found to be localized on one of the facets that was opposite the boron atom in a substituted position, and which bonded mainly with one apical boron atom. The main descriptors of the B-H^fac bond were established theoretically using QTAIM and NBO approaches. In addition, the mechanism of [2,6-B₁₀ H₈ O₂ CCH₃ ]⁻ protonation was investigated.