It has been demonstrated recently that the diffusive shock acceleration theory based on the assumption that the velocity distribution function of particles is close to isotropy is incapable of explaining observed spectra of ions accelerated at shocks in corotating interaction regions. In the present paper the shock drift acceleration of protons suffering multiple reflections at shock fronts is studied as applied to the forward and reverse shocks detected at Ulysses on day 20, 1993, and on day 22, 1993. The velocity distribution function is considered to be anisotropic near the shock fronts, and the magnetic moment of particles is supposed to be an adiabatic invariant which is valid in the case of weak scattering. ¿ 2001 American Geophysical Union