Ions form important constituents of the cold summer mesopause region, since their tendency to form heavy clusters makes them a strong candidate for the nucleation cores of the ice particles that make up noctilucent and polar mesospheric clouds and since their chemical composition can yield important information about the thermal structure and the minor neutral composition of the mesopause region itself. The sources of mesospheric ionization at high latitudes and the outlines of the chemical reaction chains that determine the steady state positive ion composition are discussed. A one‐dimensional model is used to calculate the steady state composition, and the main features are discussed, with emphasis on the altitude distribution of the proton hydrates and their dependence on such atmospheric variables as the temperature, the ionization rate, and the concentrations of water vapor and atomic oxygen. The model predictions are compared with mass spectrometer measurements made at high latitudes in summer, and the main discrepancies are pointed out. The possible role of negative ions is discussed briefly.