  The origin of bistable solutions in the kinetic equations describing the
chemistry of dense interstellar clouds is explained as being due to the
autocatalysis and feedback of oxygen nuclei from the oxygen dimer (O2). We
identify four autocatalytic processes that can operate in dense molecular
clouds, driven respectively by reactions of H+, He+, C+, and S+ with O2. We
show that these processes can produce the bistable solutions found in previous
studies, as well as the dependence on various model parameters such as the
helium ionization rate, the sulfur depletion and the {{{H}}}3+ electron
recombination rate. We also show that ion-grain neutralizations are unlikely to
affect the occurrence of bistability in dense clouds. It is pointed out that
many chemical models of astronomical sources should have the potential to show
bistable solutions.
