Abstract: A continuous measuring system for chlorine in a non-polar medium has been developed. Dilute (aqueous) methyl orange solution flows past a (Teflon) membrane mounted In a sensor. Chlorine diffuses through the membrane and reacts with methyl orange, causing decolorization. The extent of decolorization of the solution is measured colorimetrically. A linear dependence of the extinction on the chlorine concentration has been found. Chlorine concentrations of 5-200 mol/m3 at 20-40°C are measured with a standard deviation of the experimental points of 2%. The time constant (63% approach to equilibrium) is 22 s; the dead time is 30 s in the system used. Calibration of the sensor should be carried out once a day.