This work describes the influence of gaseous chlorine on thin porphyrin films. LB films of metal and free base porphyrins based on meso-tetra (4-amino sulfonyl) phenylporphyrin [1] exhibit intense optical absorption bands in the region 400-700 nm. The change in the molecular environment of the porphyrin which arises as a result of exposure to the sensor gas is reflected in substantial changes to the optical absorption spectrum. This effect is reversible in the absence of the sensor gas. In-situ optical measurements have been performed to quantify the response of the films ina stream of gas of specified concentration and their subsequent recovery in air. Factors affecting the response such as choice of porphyrin, orientation of porphyrin molecules within the LB film, concentration of sensor gas and temperature have been studied.