There have been numerous attempts to use ultraviolet photoemission spectroscopy (UPS) to monitor the chemical states of adsorbed gas molecules on metal surfaces. To interpret the data correctly, one has to determine the effect of photoemission on the measured energy levels of the molecule. We have measured the UPS spectra of seven gases (C6H6, C5H5N, CH3OH, C2H5OH, H2CO, H2O, NH3) condensed on a LN2 cooled MoS2 substrate at hν = 21.2 eV. The inertness of the MoS2 substrate assures that no strong chemical bonding exists between the substrate and adsorbed molecules. For each gas, the spectrum of the condensed phase is similar to the corresponding spectrum of the gas phase except all the energy levels are shifted up by the same amount. This shift ranges from 1 to 1.65 eV for the gases studied. The energy shift is attributed to the dielectric screening of the hole produced during the optical excitation.