The magnetic isotherms of various compounds of the type RCo5 in which Co is partially replaced by either Cu or Ni were studied at 4.2 k as a function of Co concentration. The form of the virgin magnetization curve, the presence of large intrinsic coercive forces and the observation of pronounced thermomagnetic history effects indicate the presence of Bloch walls in these compounds having a width of only a few interatomic distances. It is argued that the presence of these narrow Bloch walls is the primary reason for the high coercive forces observed usually in solid pieces of materials of the type RCo5x Cu 5−5x.