Ammonia (NH 3 ) is classified as a toxic air pollutant but its release from vehicles is not regulated. Herein we report on the efficiency of the catalytic reduction of nitrogen monoxide (NO) and the selectivity towards NH 3 . Chemical ionization mass spectrometry (CIMS) has been applied to monitor NH 3 and NO emissions at a time resolution of 2s. At real world driving, intense, catalyst-induced NH 3 formation was detected for a gasoline-fueled passenger car equipped with a Pd/Rh-based three-way-catalyst (TWC). Post-catalyst NH 3 emissions strongly depend on velocity and acceleration and varied by two orders of magnitude from 1 to 170mgkm −1 . For most vehicle conditions, tail-pipe NH 3 emissions exceeded those of NO. Excellent NO conversion above 95% was noticed as soon as catalyst light-off occurred. Post-catalyst NO emissions were lowest when NH 3 formation was most intense and vice versa. This complementary behavior indicates that a TWC can be operated in a way that either NH 3 or NO emissions dominate. The NH 3 /NO mixing ratio was mainly influenced by the air-to-fuel ratio. At fuel-rich combustion (λ<1), highest NH 3 /NO mixing ratios clearly above one were observed, whereas ratios at or below one were found at lean conditions (λ>1). Catalyst temperature effected the selectivity of the DeNOx process. Highest NH 3 selectivity up to 0.45 was found when operating the catalyst below 280°C. Above this temperature, the selectivity was reduced to 0.02–0.05. The obtained results highlight those parameters, influencing the NH 3 output of a TWC vehicle at real world driving.