The invention relates to a catalytic process for reducing nitrogen oxides in flue gases, in which process a reducing agent containing composition is mixed with flue gas and the mixture achieved is brought into contact with a catalyst. In accordance with the invention, the composition contains e.g. 1 to 60% of weight of ammonium formate. Especially a low freezing point can be achieved with such water containing composition. In addition, a good reduction conversion is achieved even at low temperatures.