An empirical study of corotating interaction regions (CIRs) observed between 3.9 AU and 5.9 AU on Pioneers 10 and 11 shows that the main corotation energetic ion population (CEIP), which is associated with the trailing reverse shock, terminates within the CIR at a definite, structural boundary, which we show here is the stream interface. This new result has significant implications for solar wind and energetic particle modeling. In particular it implies either that the reverse shock forms closer to the stream interface than models suggest or that the theories that treat the generation and transport of these energetic ions, such as preshock Fermi acceleration and cross-field diffusion must be combined or extended. We test these scenarios by comparing the CEIP intensity profiles on the two sides of the stream interface. We find that while each automatically accounts for one or two aspects of the results none of them alone can account for all of our empirical results. ¿ American Geophysical Union 1994