Ions in the outer magnetosphere typically are observed to consist of a hot (T~10 keV) component and a cool constituent with energies of a few eV. The hot component often is observed to be anisotropic with T⊥>T∥, where the subscripts denote directions relative to the background magnetic field, whereas the cool ions exhibit a wide variety of anisotropies. The Los Alamos magnetospheric plasma analyzer (MPA) instruments measure ion and electron velocity distributions from about 1 eV to about 40 keV at geosynchronous orbit. In the afternoon and evening sectors MPA observations show that cool ions, which are assumed to be protons, are sometimes warm with temperatures of order 10 eV and T⊥>T∥. Theory and simulations of the electromagnetic proton cyclotron instability, which is driven by the hot proton anisotropy, have shown that the resulting enhanced field fluctuations heat initially cool protons. Moreover, this process implies a scaling relation for the apparent temperature of the warm protons as a function of the relative densities of the two components. This manuscript describes an examination of MPA data which shows that this scaling relation provides an approximate upper bound for a selected set of warm proton temperatures observed at geosynchronous orbit. This result is consistent with the hypothesis that the proton cyclotron instability is an important energization source for warm protons at geosynchronous orbit.¿ 1997 American Geophysical Union