A supersonic purely chemical continuous‐wave HCl laser is described in which nonequilibrium concentrations of atomic chlorine are generated by the branch‐chain reaction of NO with ClO2. The ClO2is injected into a stream of NO in helium immediately upstream of the throat of a slit nozzle. The reacting gases then expand as a free jet into a low‐pressure region where HI is added to rapidly form vibrationally excited HCl. A maximum HCl multiline output power of 35 W is obtained, which represents a chemical efficiency of ∼4.5% based on the total exothermicity of the pumping reaction and the limiting flow rate of HI. An efficient technique for visualizing the flow in the expanded region is described. This technique is based on the visible chemiluminescence resulting from the addition of H2S to the NO/ClO2reaction system.