The formation of Nitrous Oxide (N2O) over an aged three way catalyst was analysed in a laboratory reactor for a variety of simulated Otto engine exhaust gas conditions. Nitrous Oxide formation was further analysed during FTP75 dynamometer test with a car. The car was equipped with either an aged catalyst or a fresh one. A fast response diode laser system was modified to enable detection of Nitrous Oxide and Carbon Monoxide simultaneously.From laboratory data the kinetics of Nitrous Oxide formation were evaluated with mathematical simulations and a mechanism was suggested. The results were compared to data from vehicle tests and the results were discussed in the light of the laboratory study. Two general trends were confirmed, i) N2O formation increases at slightly lean conditions: ii) catalysts with a low degree of deterioration gave lower N2O emissions, iii) the extent of N2O formation goes though a maximum with respect to dissociation rate of NO.