The formation of vibrational strong coupling states was confirmed using infrared (IR) transmission spectroscopy, and representative IR spectra are shown in Figure . When the cavity mode was matched to the O-H stretching frequency of water (ωst,OH = 3400 cm -1 ), the original absorption peak disappeared, and two distinct peaks corresponding to vibro-polaritonic states emerged. This phenomenon is consistent with previous reports on the vibrational strong coupling of water. The anti-crossing behavior of the upper and lower polariton branches was evaluated, as shown in Figure . The frequency of the third-order cavity mode was adjusted by controlling the cavity thickness within the range of 2.8 to 4.0 μm. We investigated electrolyte solutions of LiCl, NaCl, KCl, RbCl, CsCl, Me4NCl, Et4NCl, n Pr4NCl, n Bu4NCl, LiF, and LiBr. The quality factor of the fourth-order cavity mode varied between 22 and 25 in each independent experiment. Even in the presence of electrolyte, the observed anti-crossing behavior remained consistent. The consistent vibro-polaritonic behavior was observed for combinations of alkali cations, tetraalkylammonium cations, or halide anions. This is rea-sonable because the concentration of the electrolyte (0.01 M) is significantly lower than the concentration of water (55.5 M). The Rabi splitting frequency was estimated when the cavity mode was tuned to the O-H stretching frequency, resulting in a determined value of 760 cm -1 . This value is similar to reports on vibrational polaritons in pure water. Under strong coupling conditions, the optical mode was confined within the cavity, enabling coherent interactions with the vibrational mode of water. Under these conditions, ωR exceeded the sum of the photon loss rate of the cavity and the dephasing rate of the O-H stretching frequency of water molecules. The photon loss rate of the cavity mode was estimated as 200 cm -1 , and the homogeneous line width of the O-H stretching mode, corresponding to the dephasing rate of vibrational modes, was reported as 120 cm -1 . The relationships between Rabi splitting energy, photon loss rate, and dephasing rate provide support for the clear formation of vibrational strong coupling states of water molecules in the cavity, even in the presence of an electrolyte. This system achieved a particularly strong vibrational ultra-strong coupling, where ωR / 2ωst,OH = 0.11, serving as a benchmark among molecules coupled to vacuum fields.