Stable steady states of tank reactors belonging to the group of multiple states are separated by unstable slates. Are separated by unstable ones. Unstable states, however, do not form the boundary of the regions of attraction trajectories. It became evident that even for the simplest type of the reactor, namely an adiabatic homogeneous tank reactor, the boundary is given by a certain separating curve. The objective of the present study is to determine this separatrix.