It is common to view the expansive phase of a magnetospheric substorm as a large amplitude disturbance. As a result, any magnetic disturbance that has the normal characteristics of an expansive phase onset but has an amplitude below some subjective limit is not classed as an expansive phase onset but is labelled, instead, as a pseudo-breakup. In this paper it will be shown that a pseudo-breakup is, in fact, a disturbance which represents the same physical process as the onset of an expansive phase or any of the subsequent intensifications. Examination of simultaneous interplanetary magnetic field data suggests the possibility that a pseudo-breakup is distinguished from the onset or intensifications of an expansive phase by its occurrence during a period of increasing energy input into the magnetosphere. Thus, the pseudo-breakup is not accompanied by a large scale poleward motion of the high latitude edge of the auroral oval in the substorm disturbed region characteristic of an expansive phase. ¿ 1998 American Geophysical Union