The emissions of nitrous oxide and methane from a Pratt & Whitney Canada PW 305 and a Rolls Royce RB 211 jet engine were measured under various flight conditions either on a ground level stationary test stand or in altitude test cells by using an off-line sampling technique. The concentrations of the gases were determined by long path infrared diode laser absorption spectroscopy in the laboratory. The calculated emission indices indicate that, at present, air traffic doesnot contribute significantly to the global budgets of methane and nitrous oxide. ¿ American Geophysical Union 1994