AMPTE/IRM measurements during a period of strong magnetic activity on April 8, 1985 exhibit a significant hardening of the energy spectra of all detected ion species (H+, He2+, He+, O+) following substorm expansion. In order to explain these observations in the framework of the substorm neutral line model, single-particle trajectories of various ions (H+, He2+, O+) are followed numerically in the time-dependent electric and magnetic fields obtained from a two-dimensional magnetohydrodynamic simulation of magnetotail reconnection. Current sheet acceleration of a suitable plasma sheet ion distribution can explain the observed ordering of the flux increase of the various species in energy per charge. Starting from a cold lobe population, however, current sheet acceleration cannot account for the differential ion intensities observed after substorm expansion. ¿ American Geophysical Union 1989