We proposed a new separation technique for trace volatile organic compounds (VOCs) from gases (Enuiron. Sci. Technol. 2002, 36, 4170-4174) that utilizes the selective ionization of VOCs and electrical migration of ions in bifurcating flow. In the present work, we employed a soft X-ray source for the ionization of VOCs and investigated the effect of the X-ray intensity and VOC concentration on the separation efficiency. As a result, experimental results showed that a higher separation efficiency is attained with a higher intensity of the soft X-ray source and that the separation efficiency increases with decreasing the VOC concentration. The dependency of the VOC separation efficiency on these factors is well explained by the proposed separation model, which accounts for the reactions of VOC intermediates and reactive primary cations of carrier-gas molecules.