Abstract: A method and device for estimating temperature of gases arising from internal combustion of an engine at the entrance to a system for post-treatment of the gases. The post-treatment system is downstream from a system for treatment of the gases, which may include a nitrogen oxide NOx trap, an oxidation catalyst, or a 4-way catalyst. The systems connect to each other by a line, wherein the treatment system is considered as the whole of a whole finite number n of fictive elementary reactors Ri that are perfectly agitated and serially connected, wherein the total number n of fictive elementary reactors Ri is determined, in particular as a function of volume of the treatment system, and the fictive elementary reactors Ri are numbered from 1 to n in the gas flow direction along the treatment system.