A chemical gas-phase reaction between Cl2 and HI was used in the generation of molecular iodine for the chemical oxygen-iodine laser (COIL) operation. A yield of I2 in the generation reaction up to 85% was achieved in a reasonable volume of the reactor. A small-signal gain up to 0.75%/cm at a temperature of 150 K in the center of supersonic cavity was measured. A comparison with the established evaporation way of I2 delivery confirmed that the chemical method has little or no impact on the COIL pumping kinetics. This chemical method is easily scalable and can simplify the COIL operation by providing better control of I2 flow rate.