Abstract: A photo-ionization detector utilizes an ultraviolet (UV) lamp for detecting and measuring the concentration of volatile gases flowing between closely spaced parallel electrodes. One of the electrodes is made of mesh to allow photons to pass into the space between the electrodes to ionize the volatile gases between the electrodes. The detector also incorporates an improved ionization chamber. In other embodiments, a plurality of gas discharge lamps, each generating a different photon energy, may be placed adjacent to a plurality of closely spaced electrodes, all electrodes in one ionization chamber, to detect and measure different types of volatile gases that may exist.