In this work we report how the use of low-sulfur gasoline decreases formation of N2O as a by-product during light-off of commercial Pd-only three-way catalytic converters (TWC). Our results also show that even in the absence of sulfur a large quantity of N2O (reaching instantaneous concentrations up to 700ppm) is produced at low temperature, between 200 and 400C. At high temperature (400 to 600C) and lean conditions we found an unexpected emission of N2O (200ppm), which is apparently produced by reduction of NO with C3H8 on the TWC. The emission of N2O during light-off is practically independent of the air-to-fuel ratio but depends strongly of the sulfur content in gasoline. Under rich and stoichiometric conditions, the presence of SO2 promotes N2O formation (concentrations from 700 to 900ppm) via NO reduction by CO. The presence of SO2 in a lean environment has no effect upon formation of N2O.