Acidic ionic liquid (AIL) solutions were prepared by dissolving bis(trifluoromethanesulfonyl)imide (HTFSI) acid in the ionic liquid (IL) 1-butyl-3-methylpyrrolidinium bis(trifloromethanesulfonyl)imide (PyrrTFSI). The HTFSI/PyrrTFSI solutions were investigated by conductivity measurements, optical spectroscopy, and DFT calculations in order to understand the ionization/solvation mechanism of HTFSI in the solutions. The HTFSI/PyrrTFSI solution conductivities first increased at lower concentrations and then decreased when the concentration of HTFSI is higher than ∼1.5 M. The spectroscopic results indicate that the solvation structure may evolve from lower to higher concentrations to make protonated TFSI(-) motifs. Both spectroscopic and DFT simulation results support the observation of proton-sharing [H(TFSI)2](-) dimers, which may form through a bridged hydrogen in the format of either a N-H-N connection or a N-H-O connection. Both configurations may exist in the AIL solution. The proton-sharing mechanism implied by these structures confirms that the TFSI(-) ion can be a proton acceptor and a Brønsted base as well in IL solutions. However, the IL molecular cations such as imidazolium and (in this work) pyrrolidinium do not contribute significantly to the proton solvation and transportation in the solutions.