We examine the performance of two nonlinear feedback trajectory tracking controllers designed to achieve the regulated transfer from a stable operating equilibrium point towards a more efficient, yet open loop unstable, equilibrium point in a nonlinear multivariable exothermic chemical reactor system. A passivity based controller, designed with reasonable passive outputs reference trajectories, is shown to be unsuitable, dueto unrealistic reactant concentration and temperature controller profiles in the unrestricted controls case, or, due to controller saturation, and convergence to undesirable equilibria, when hard constraints are imposed on the control input signals. However, an exact tracking error linearization controller, designed with similar reference trajectories for the flat outputs is shown to be entirely feasible in the unrestricted controls case and, furthermore, devoid ofcontrol signals saturations in the restricted case. The differential flatness property is then used to synthesize a suitable feedback passivity based controller with passive outputs reference trajectories indirectly specified in terms of the flat output trajectories. The obtained “passivity plus flatness” (PF) controller is shown to be entirely equivale to the “linearizing plus flatness” (LF) controller when plant parameter perturbations and initial state setting errors are absent from the equilibrium transfer maneuver. However, when such perturbations are present, the LF controller is shown to be superior to the PF controller in several respects.