The interaction of the solar wind with gyrophase bunched ions reflected at an oblique (&THgr;Bn=45¿) shock (MA=8) is simulated using a 1D hybrid code. These ions are unstable to low frequency hydromagnetic waves as well as whistler waves. At some point in the hydromagnetic wave growth ion trapping occurs which restricts the local distribution in gyrophase to a higher degree than the input distribution. Given the growth rate of the MHD waves, gyrophase mixing is ineffective is isotropising the ions, except at the leading edge of the injected distribution. ¿ American Geophysical Union 1993