Measurements of reversible and irreversible magnetization have been used to examine the reversal process on the initial magnetization curve in sintered NdFeB. It is found that the reversal process involves equal and opposite changes of irreversible and reversible magnetization, described here as isomagnetic reversal, resulting in no net change in magnetization during reversal. This result is consistent with the observation that magnetic viscosity is negligible on the initial magnetization curve. (C) 1999 American Institute of Physics. [S0003-6951(99)00504-5].