Abstract The satellite Ariel I, launched in April 1962, provided a survey of the topside ionosphere over a latitude range of ± 55°. The measurements made by the ion energy spectrometer showed the ions O+ and He+ to be the major massive components of the ionosphere and enabled a global study of the composition over the northern summer of 1962 to be made. Publication of earlier analyses of parts of the data showed a diurnal and seasonal variation in the composition, a strong geomagnetic control and a suggestion of a departure from hydrostatic equilibrium in the diffusive separation of the ions. This paper provides a definitive presentation of the computer analysis of nearly all of the ion composition data obtained by the energy spectrometer. It confirms and strengthens the earlier conclusions by the inclusion of much more data and by its regression analysis in terms of geomagnetic latitude, altitude and local solar time. The paper also gives results of the total ionization density measurements and of a study of the effect of vehicle aspect on these data.