The reactions leading to N2O formation or destruction during NO reduction by CO on a Pt-Rh catalyst have been studied, and their rate constants have been experimentally determined. N2O concentrations in real exhaust gases were then measured on an engine test bench as a function of rotation speed, torque and EGR rate. The effect of a three-way Pt-Rh catalyst was determined as a function of exhaust gas temperature or air-fuel ratio. The catalyst increases N2O concentration at temperatures just after light-off. Above 480°C it tends on the contrary, to lower its concentration slightly. N2O emissions were also measured on two vehicles and found to be higher with a three-way catalyst than without. The difference is only great however when the test is performed at, low speed after a cold start (i.e. ECE urban cycle), but not after a hot start or on a faster test cycle (i.e. FTP or ECE extra-urban cycles).Some comparisons with diesel engines have also been made.