Data from the S3-3 and DE 1 satellites are analyzed to study the interaction between H+ and O+ ions in upgoing auroral beams. Every data set analyzed showed some evidence of an interaction. The measured plasma was found to be unstable to a low-frequency electrostatic wave that propagates at an oblique angle to B↘0. A second wave, which can propagate parallel to B↘0, is weakly damped in the plasma studied in most detail. It is likely that the upgoing ion beams generate this parallel wave at lower altitudes. The resulting wave-particle interactions qualitatively can explain most of the features observed in ion distribution functions. H+ ions in the low-velocity tail of the H+ beam are decelerated and O+ ions in the high-velocity tail of the 0+ beam are accelerated through this process. This may explain the occasional observation of O+ beams with substantially higher streaming energies than the simultaneously observed H+ beams. The wave-particle interactions also can explain the asymmetrical features that regularly are seen in H+ and 0+ distribution functions. The wave which was found to be unstable in the measured plasma primarily accelerates 0+ perpendicular to BO. This process can produce weakly conical 0+ distributions which are characterized by T⊥>T&Pgr; rather than by having a peak at 90¿ pitch angle within the acceleration region. In general, the interaction between different ion species in multicomponent beams appears to be a dominant factor in determining the structure of upgoing auroral ions. The same process is likely to be important in a number of other beam-containing space plasmas.