We present the results of a parameter study of the influence of heavy ions on the background solar wind, choosing doubly ionized helium, or alpha particles, and O+6, as examples. Using a three-fluid solar wind model, we keep the input parameters to the electrons and protons unchanged and investigate the effects of changing the input energy flux to the heavy ions and their coronal abundance, i.e., their abundance at 1 Rs, on the background electron-proton solar wind. Our results confirm earlier studies that alpha particles can have a dramatic effect on the thermodynamic and flow properties of the protons in the solar wind. The maximum coronal abundance for which the changes in the energy input to the heavy ions has no effect on the protons is 5¿10-4 for the alphas and 5¿10-5 for the oxygen ions, which are well below the photospheric values. For larger coronal abundances, the sensitivity of the changes of the flow speed and proton mass flux to changes in the energy input to the heavy ions increases sharply with increasing abundance. When the heavy ions are not heated, the increase in the coronal abundance leads to an increase in flow speed, a decrease in proton mass flux, and an increase in proton temperature at 1 AU. However, as the heat input to the heavy ions increases, the dependence of these parameters on the abundance goes through a transition and starts to follow the opposite pattern, namely a decrease in flow speed and proton temperature at 1 AU, and an increase in proton mass flux. This study shows that, for currently known photospheric elemental abundances, the flow properties of heavy ions cannot be investigated independently of those of the bulk proton-electron solar wind. The effect of heavy ions on the electron-proton bulk solar wind is determined primarily by the collisions occurring very close to the coronal base. Hence including physical processes responsible for the preferential heating of heavy ions to temperatures exceeding those of protons in the inner corona cannot be done without considering the subsequent implications for the protons and electrons in a self-consistent manner.¿ 1997 American Geophysical Union