The temporal variation of the solar flare particle alpha to proton ratio during energetic storm particle events is investigated within the framework of a model where a particle gains energy by successive reflections at the moving interplanetary shock front. The basic parameter which produces ratio changes is the energy dependence of the interplanetary diffusion coefficient and by inference the power spectral slope of the fluctuations in the interplanetary magnetic field. For a power spectrum of the type P~f-n with n=1 the diffusion coefficient is proportional to rigidity times velocity, and the alpha to proton ratio is therefore constant at equal energy per change. Monte Carlo simulation of the particle-shock interaction shows that even for power spectral slopes as high as 4/3 ratio at equal energy per charge not vary greatly during the event. It is concluded that composition changes during energetic storm particle events reflect (at least in part) the power spectral slope n of the magnetic fluctuations responsible for the particle scattering.