Ion composition measurements have shown that the dominant ions are mainly proton hydrates (PH) or (H+ (H2O)n) in the upper stratosphere and mesosphere and nonproton hydrates (NPH) (or H+ X(H2O)m) in the lower stratosphere. Theoretical and experimental studies have indicated that the identify of X is almost certainly methyl cyanide (CH3CN). This paper reports a two-dimensional model of ion composition from 15 km to the stratopause. The model accounts for the latitudinal variation in the ionization rate by galactic cosmic rays and in the concentration of neutral species which affect ion chemistry. The results obtained indicate several new and interesting features. The altitude of the crossover point of PH over NPH is found to vary with latitude. The PH ions start to dominate as low as 27 km near the poles, whereas at the equatorial latitude, NPH ions remain the dominant ions up to about 45 km. The modeled profiles at 45 ¿N, the only latitude at which experimental data are available, conform very well to those data. The ion composition is found to be highly dependent on the reaction rate coefficient (K) for the conversion of NPH to PH, which is poorly known. The meridional distribution of ion composition and a comparison with the available observational data is presented. Finally, the need for future ion composition measurements at other latitudes is emphasized. ¿ American Geophysical Union 1992