The evolution of tropospheric ions has been studied by investigating the change in the mobility spectrum of ions generated in laboratory air as a function of aging time. An ion mobility spectrometer with a drift tube was constructed to get high-resolution measurements of mobility spectra of ions aged for about 0.03-5 s. Mobility spectra and their changes with ion age obtained in the present study showed the following features of tropospheric ion evolution near the ground: The positive ion mobility spectrum changes significantly with aging time, in which the peak mobility value shifts from 1.8 to about 1.0 cm2 V-1 s-1. Six peaks are usually distinguished in the mobility range between 2.0 and 1.0 cm2 V-1 s-1 in the positive spectrum, and a tendency of positive ions to evolve to have lower mobilities with increasing ion age is observed. Four additional peaks are sometimes resolved at around 0.9 cm2 V-1 s-1, indicating formation of heavy positive ions (probably over 400 amu). The negative ion mobility spectrum obtained is characterized by two prominent peaks at 1.9 and 1.7 cm2 V-1 s-1. The relative intensity of the two peaks varies with ion age, and the ions with a mobility of 1.9 cm2 V-1 s-1 dominate after about 1 s. Further conversions of the negative ions to those with lower mobilities can be expected from a comparison with the natural ion spectra obtained so far by the Gerdien type aspiration condenser. ¿ 1998 American Geophysical Union