The knowledge of the dew point of compressed humid gases is needed for many new technical applications as, for example, the compressed air energy storage or the humid air turbine. The dew point concentration of water in compressed gases deviates with increasing pressure from the well-known ideal gas model. This deviation can be described either by the vapor pressure or the vapor concentration enhancement factor. There are only very few experimental data in a limited range available for the first factor and none at all for the latter. Here, a new method was developed and a new apparatus has been constructed to measure the vapor concentration enhancement factor by Fourier transform infrared spectroscopy. The apparatus is limited to a temperature range from (10 to 150) degrees C and pressures up to 25 MPa. The combined standard uncertainties (confidence level 68 %) of the new experimental data are 0.02 K for the temperature, 3.2 kPa for the pressure, and (0.13 to 1.15) % for the vapor concentration enhancement factor.