We have observed the photo-chemical reaction process of organic molecules in gas phase induced by vacuum ultraviolet (VUV) radiation. We chose hydrocarbons that contain five carbons in a molecule as samples. To observe the wavelength dependence, three kinds of excimer lamps (Ar-2*-126 nm, Kr-2*-147 nm and Xe-2*-172 nm) were used as light sources of the VUV radiation. Vaporized organic molecular compounds were irradiated by the VUV at a pressure of 12 Pa and detected by means of mass spectrometry. Change of the mass spectra with the irradiation period was measured. As a result, we found that different reaction rates were observed for the different compounds. In particular, 1-pentene and cyclopentane, which have the same molecular weight, have very different decomposition speeds for the irradiation of 172-nm radiation. The facts observed in this work are considered to be very promising for developing a technique to distinguish the targeted compound from others that have similar structures. (C) 2009 Elsevier B. V. All rights reserved.