Gas reburning is a NOx control technique that can be applied in different combustion systems. In it, gas is injected into the furnace downstream of the primary burners to form a substoichiometric zone followed by the injection of burnout air. The variables that influence the chemistry in the reducing zone (temperature between 900 and 1450 K, reburn fuel type, stoichiometry, NO concentration and residence time), which is mainly controlled by the interactions between hydrocarbon radicals and NO, have been studied in a laboratory scale flow reactor. A significant NO reduction efficiency can be obtained with proper operating conditions. The NO destruction is closely coupled to the hydrocarbon oxidation regime, which differs depending on the hydrocarbon fuel considered, and NO is to a significant extent converted to HCN. The concentration of hydrocarbons and oxygen, and the temperature are important parameters affecting the results obtained, whereas the inlet NO concentration and the residence time only have a minor effect. The practical implications of the present results for gas reburning are discussed.