A two ion species system of fluid equations is used to investigate mechanisms responsible for transverse accelerations of plasma releases into high Mach number flows, such as that of the December 1984 Active Magnetospheric Particle Tracer Experiment (AMPTE) release of barium ions into the solar wind. We extend previous work in single ion species systems by developing a set of modified MHD equations which incorporates both the unmagnetized nature of the released ions and the interactions of the two ion fluids. An analysis of this system of equations suggests two mechanisms that could cause transverse shifts of the plasma release. One of these is based on a magnetic drift wave, the other on a hybrid cyclotron mode. Numerical simulations produce clearly observable shifts in the case of each mechanism. The modified two-fluid MHD equations, simulation results, and physical mechanisms are discussed, and their relevance to the AMPTE barium release is examined. ¿ American Geophysical Union 1994