Total positive chemiion concentrations n in jet engine exhaust at the ground were measured using an electrostatic probe. Data were obtained for two different engines (RR M45H and JT9D-7) for one power setting (half thrust), two fuel sulfur contents (2 and 66 ppmm), and four distances d (1.39, 2.19, 4.19, and 18.6 m) behind the engines. The corresponding plume ages tp ranged from about 12 to 170 ms. As tp increases n decreases steeply from 1.6¿108 cm-3 at tp=12 ms to 3¿106 cm-3 at tp=170 ms. We conclude that this decrease mostly reflects ion removal by ion-ion recombination with a mean rate coefficient of about 7.5¿10-7 cm3/s. Our experimental findings have interesting implications for the chemical evolution and potential roles of chemiions in jet engine exhaust plumes. These implications are briefly addressed. ¿ 2000 American Geophysical Union