A microwave cavity method has been used to measure the electron loss rate following a pulsed discharge in oxygen-nitrogen mixtures. Preliminary results indicate that, when suitable pressures of oxygen and nitrogen are chosen, the main mechanism of electron removal in the later afterglow is a three-body attachment process. The three-body coefficient K=2.4 × 10^-30 cm⁶ sec^-1 is in good agreement with the drift tube measurements, for thermal electrons, of Chanin, Phelps and Biondi.