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A new ion source (IS) utilizing vacuum ultraviolet (VUV) light is developed and characterized for use with iodide-chemical ionization mass spectrometers (I−-CIMS). The VUV-IS utilizes a compact krypton lamp that emits light in two wavelength bands corresponding to energies of ~10.0 and 10.6 eV. The VUV light photoionizes either methyl iodide (ionization potential, IP = 9.54 ± 0.02 eV) or benzene (IP = 9.24378 ± 0.00007 eV) to form cations and photoelectrons. The electrons react with methyl iodide to form I− which serves as the reagent ion for the CIMS. The VUV-IS is characterized by measuring the sensitivity of a quadrupole CIMS (Q-CIMS) to formic acid, molecular chlorine, and nitryl chloride under a variety of flow and pressure conditions. The sensitivity of the Q-CIMS, with the VUV-IS, reached up to ~700 Hz pptv−1, with detection limits of less than 1 pptv for a one minute integration period. The reliability of the Q-CIMS with a VUV-IS is demonstrated with data from a month long ground-based field campaign. The VUV-IS is further tested by operation on a high resolution time-of-flight CIMS (TOF-CIMS). Sensitivities greater than 25 Hz pptv−1 were obtained for formic acid and molecular chlorine, which were similar to that obtained with a radioactive source. In addition, the mass spectra from sampling ambient air was cleaner with the VUV-IS on the TOF-CIMS compared to measurements using a radioactive source. These results demonstrate that the VUV lamp is a viable substitute for radioactive ion sources on I−-CIMS systems for most applications. In addition, the VUV-IS can likely be extended to other reagent ions, such as SF6− which are formed from high IP electron attachers, by the use of absorbers such as benzene to serve as a source of photoelectrons. [ABSTRACT FROM AUTHOR]