Combustion flue gas containing NOX and SOX is treated to remove NOX in a multistep system in which NOX is reduced in the flue gas stream via selective catalytic reduction or selective non-catalytic reduction with ammonia or an ammonia-forming compound, followed treatment with hydrogen peroxide to remove residual ammonia and, optionally, treatment with an alkali reagent to reduce residual NOX in the flue gas stream. The NOX-depleted flue gas stream may also be subjected to a desulfurization treatment for removal of SOX.