An optical measuring head for a duct gas monitoring system is provided, the measuring head being mounted to an outer wall of a gas duct through which the duct gas flows. The measuring head has a longitudinal chamber which at one end opens into the gas duct and at the other end contains an active optical component. The chamber is flushed with a purge gas which, after flushing the chamber, is discharged into the gas duct. A gas line is installed between the chamber and interior of the gas duct at a point upstream of a discharge point and the purge gas is a branch-off of the duct gas.