The chemical composition of small ions has been studied by means of formal chemical kinetics. The model for negative ions contains 474 ion-molecular reactions, 143 ions, and 106 neutral compounds, and the model for positive ions contains 1044 reactions, 198 ions, and 131 neutral compounds. Both the time variation of individual ion species and the abundances of ions in the steady state have been calculated. The authors have used an original method for solving large and stiff systems of differential equations. The results of calculations have been compared with the results obtained both by mass spectrometry and by mobility spectrometry. The ions of an age of about 1 ms are similar on the basis of both the measurements and the modeling. The same (with certain reservations) is true of the ions in the steady state. Certain discrepancies appear in the case of the ions with an age of about 1 s. A lack of data concerning several important reactions is still evident. A special study showed that the concentration of H2O, O3, NO, NO2, and HNO3 significantly influence the evolution of negative ions; NH3 has a strong effect on the evolution of positive ions. ¿ American Geophysical Union 1994