A statistical study of the composition of the plasma sheet at geocentric radial distances between 10 and 23 RE has been conducted utilizing data from the energetic ion mass spectrometer experiment on ISEE 1. The composition of the ions in the energy per charge range 0.1≤Elq≤16 keV/e was found to be dependent on substorm activity. The O+/H+ and He+/H+ ratios of the ions increased and the He++/H+ ratio decreased with increasing AE. O+/H+ exhibited the largest changes, varying from about 2% during periods of extended quiet to about 40% shortly after a major substorm. H+ was typically the major constituent, but on some occasions O+ was dominant. Assuming that the composition of the ionospheric and solar wind supplied ions to the plasma sheet does not vary systematically from quiet to active times, inferences are drawn as to the relative importance of the ionosphere and solar wind as sources of the plasma sheet ions. The results suggest that during quiet times the plasma sheet is almost entirely of solar wind origin while during active times the ionosphere is of comparable importance to the solar wind. The average energies of the ions of solar wind origin appear to be significantly increased in association with substorm activity, while the average energies of the ions of ionospheric origin show little or no effect. The solar H+ component appears to be more strongly energized than does He++.