Ion/ion instabilities are driven by the relative velocity v0 of two distinct ion components. This manuscript reports linear theory and simulation studies of electromagnetic ion/ion instabilities for a warm proton component streaming against a cool, heavy ion component in a homogeneous, magnetized, collisionless plasma in which v0 is parallel or antiparallel to the background magnetic field. Under these conditions linear Vlasov dispersion theory predicts that the fastest growing mode is usually the right-hand polarized proton resonant instability. Two-dimensional initial value hybrid simulations of this growing mode are carried out, yielding scaling relations for the maximum fluctuating magnetic field energy density and the proton pickup time. The latter is proportional to the inverse of the maximum growth rate for a wide range of plasma parameters. ¿ 1999 American Geophysical Union