An antifouling window assembly in a duct gas monitoring system comprises a mixing chamber positioned between a duct opening and a window unit. Purge gas is introduced into the mixing chamber through an inlet located adjacent the window unit. The velocity of the purge gas is such that laminar flow is established along the surface of the window unit. The purge gas circulates within the chamber and passes into the duct through the duct opening to mix with duct gases and deflect the gases from the mixing chamber.