The aim of this paper is to find reliable criterions whether the fast homogeneous decompositions of supersaturated AlZn alloys are governed by the “normal” nucleation and growth process or by the spinodal mode. Arguments are delivered that the position of the aluminium rich side of both, the chemical and the coherent spinodal calculated by means of thermodynamical methods is fairly certain. Thus indications of the start of the rapid increase of decomposition above the chemical spinodal can impossibly be related to the spinodal mode, but to the normal nucleation and growth process, which may initiate a “fast autocatalytic nucleation of zones”. This statement is supported by SAXS investigations.