Abstract: The invention relates to a device for determination of the quantity of NOx emitted by a diesel engine (10) in a motor vehicle with common-rail fuel supply means (12) for the cylinders thereof, of the type comprising pressure recording means (32) in at least one cylinder of the engine and means (50), for determining the mass fraction of oxygen in the mixture admitted into the cylinder. Said device comprises means (58, 68), for determining a temperature of the flame front on combustion of the mixture, means (52), for determining the mass of fuel burnt in the cylinder and means (70), for calculating the quantity of NOx emitted by the combustion of the mixture in the cylinder as a function of the recorded pressure, the mass fraction of oxygen in the mixture, the temperature of the flame front and the mass of fuel burnt.