The output feedback control problem is addressed for a class of nonlinear multivariable high-index differential-algebraic-equation systems in semiexplicit form. initially, an algorithmic procedure is developed and used to derive an equivalent state-space realization of the constrained system. An output feedback synthesis problem is then formulated on the basis of the derived state-space realization and solved through the combination of state feedback and appropriate state observers. The developed methodology is applied to a two-phase reactor, and its performance and robustness characteristics are evaluated through simulations.