The average flow speed of alpha particles relative to protons in the solar wind is observed to be of the order of or less than the Alfv¿n speed. This flow, if sufficiently large, can lead to two distinct types of electromagnetic alpha/proton instabilities: magnetosonic and Alfv¿n. Spatially homogeneous two-dimensional hybrid simulations of collisionless plasmas are used to study these growing modes and the consequent wave-particle scattering of each ion component. This scattering reduces the alpha/proton average flow speed, heats the alphas more strongly than the protons, and increases the alpha temperature in the directions perpendicular to the background magnetic field, all of which are consistent with solar wind observations. ¿ 2000 American Geophysical Union