Multiple adaptive and nonadaptive models are used to represent the behavior of processes that are known to transition to unknown regimes in the operating space. The adaptive models investigated are of two types, free-running and re-initializable, that differ in the initialization of their parameters. Using these models and their companion controllers in a model reference adaptive structure, it is shown that this mixed model set can provide satisfactory control of a nonlinear, interactive, multiple-input multiple-output chemical reactor with active constraints.