H+ ion acceleration by electromagnetic ion cyclotron waves in the Earth's mid-altitude (300 to 2000 km) ionosphere is investigated. Below about 800 km, the ionospheric density is sufficiently high to induce significant collision rates between thermal particles and accelerated H+ ions. The dominant collisions are assoicated with ion-ion Coulomb interactions and with charge exchange reactions of the type H+ +A→H+A+. These reactions are able to substantially modify the space and velocity distribution of the accelerated H+ ions. Using Monte-Carlo simulations, it is shown that the combination of collisions and ion cyclotron acceleration produces energetic H+ ion distributions whose characteristics (flux, average energy and pitch angle) are in quantitative agreement with satellite observations. The distributions of the energetic H atoms which are produced by charge exchange and which might be used for remote sensing of such H+ ion acceleration processes, are also presented. ¿ American Geophysical Union 1995