Abstract: Microamounts of chlorine in air were determined photometrically by its reaction with KBr in a dilute H2SO4 medium to liberate Br2 which in turn oxidize methyl orange (MO) leading to color-fading of MO and decrease of absorbance (ΔA) measured at the wavelength of 515 nm.Factors influential to the color-fading reaction,including the amount and time of sampling,the blank test,time and temperature of the reaction,acidity of the absorbing solution and etc,were studied and optimized.