Energetic particle data from the AMPTE Charge Composition Explorer (CCE) spacecraft in the outer dayside magnetosphere are examined during the period of an upstream ion event observed by AMPTE Ion Release Module (IRM) spacecraft on September 5, 1984 [Moebius et al., this issue>. The CCE data reveal the following: (a) an ion enhancement was observed at ~0040 UT in near coincidence with a substorm onset at ~0035 UT, approximately 15 minutes prior to the onset of the event upstream of the shock; (b) ions of both solar wind (He++, Fe-group) and ionospheric (O+) origin over a broad energy range (~20 keV to >1350 keV) were injected at substorm onset; (c) the time evolution of the H+, He++, and O+ pitch angle distributions markedly differed, with O+ exhibiting mostly enhancements at off-90¿ angles for the first hour after injection; (d) an enhancement in the Fe-group ions inside the magnetosphere at L~6.4 occurred simultaneously with the appearance of an O+ burst upstream of the shock. The CCE observations, taken together with the simultaneously observed IRM ion event, suggest that a plausible explanation for the appearence of upstream ions is leakage from the magnetosphere into the upstream region, rather than the alternate explanation which requires in-situ acceleration of solar wind ions via the Fermi mechanism.