The influence of gaseous chlorine on monolayers of metal-free meso-tetra(4-amino sulfonyl)phenylporphyrin is discussed. These films exhibit characteristic intense optical absorption bands in the region 400-700 nm. Upon exposure to chlorine vapour in the concentration range 0.1-10 ppm, the Soret and Q-bands decrease in intensity and new bands at ~455 nm and ~660 nm appear. The rate of response has been measured as a function of deposition surface pressure and gas concentration. Repeated dosing experiments have shown that reversible and reproducible sensing action occurs provided that a threshold concentration is not exceeded. The recovery of the original optical spectrum can be accelerated with moderate heat treatment (60°C for 10 min). Ageing experiments have shown that the basic response of the monolayer is not effected over a time period of at least 5 months. ; http://www.sciencedirect.com/science/article/B6TW0-3VV0K4W-2D/1/d4a4ed344aa0216ef5d8648dd4c71029