The interactions between Ammoeng 100 and water are probed using high-pressure infrared measurements and DFT-calculations. The results of infrared absorption profiles suggest that the energetically favored approach for water molecules to interact with Ammoeng 100 is via the formation of anion-water interactions, whereas the alkyl C-H groups play much less important roles. After comparison with pure Ammoeng 100, it appears that no appreciable changes in band frequencies of alkyl C-H vibrations occurred as Ammoeng 100 was mixed with D2O. The presence of D2O has a red-shift effect on the peak frequency of the S=O stretching vibration under the pressures below 1 GPa in comparison to the absorption frequencies of pure Ammoeng 100. This observation is likely related to local structures of the S=O groups interacting with D2O molecules. DFT-calculations indicate that the most energetically favored conformation of ion pairs should be the species having only one hydrophilic hydrogen bonding. The results of calculations reveal that water addition may induce the partial replacement of C-H···O interactions with strong hydrogen bonding between anions and water molecules.