Abstract: In a method for photo-ionization ion mobility spectrometry, a reagent gas is added, in particular an aromatic compound, which has a large ionization cross section in the range of ionizing VUV radiation, but a low probability for the formation of protonated quasi-molecular ions. In this way, the detection of only weakly proton affine substances is also amplified or even made possible at all, and also the detection of electronegative substances in a negative operating mode is improved. Preferred reagent gases are benzene, toluene and xylene. FIG.