A magnetic sector field mass spectrometer on the Explorer XXXI satellite, when calibrated in flight against electron concentration data from the Alouette II topside sounder satellite, measures the absolute concentrations of the various positive ion species in the ionosphere. The in-flight calibration is performed by comparing the total ion current measured by the mass spectrometer, corrected for internal discriminations, with the electron concentrations, first in regions of nearly pure H+, then in regions of nearly pure O+, to obtain calibration coefficients for each ion species. Data at 500 km show as many as ten different ion species in the ionosphere. Over the polar region, H+ions streaming upward at 10 to 15 km/s have been observed.