Gas phase hydrogen bonding was studied via infrared spectroscopy in the following systems : methyl acetate-hydrogen chloride, methyl formate-hydrogen chloride, 2-butanone-hydrogen chloride, and acetone-hydrogen chloride. The intensity of the H-CL stretching vibration was monitored as a function of temperature, and the hydrogen bond energies and enthalpies were determined. The hydrogen bond energy for all four complexes was within 2.0 kJ mol(-1) of -18.3 kJ mol(-1), indicating that the hydrogen bond energy is only minimally affected by nonlocal molecular structure. The acetone-hydrogen chloride complex was modeled by ab initio methods. The energy of formation of the complex, calculated at the MP2/6-311++G** level of theory, is -20.8 kJ mol(-1), in good agreement with the experiment.